_{*This answer had to be substantially revised due to new information}

This is a multipart answer to a two part question

tl;dr:

1. It is indeed almost a direct quote to something that Marx alledgedly said that evening. But the line in the movie is quite different from what the question here implies.† The question misquotes and misattributres. The movie itself fares a bit better.

2. Given the timeframe and the circumstances surrounding that saying, there is no single person and no specific group an unquestionable candidate at whom this might have been directed. There a a number of reasonable candidates. But in the movie/quote Marx talks not about Russians but about conditions that apply in Russia.

Part 1: Trying to reconcile question and movie

The movie seems to say this:

Weitling: All power to the workers to establish universal harmony…
Just 100,000 armed proletarians, backed up by 40,000 criminals, could bring down bourgeois tyranny.
Marx: I say that stirring up the workers without offering them a constructive doctrine is a dishonest, pretentious game with an inspired prophet on one side and gaping idiots on the other!
Weitling: My audience… Idiots?
someone1: Scandalous!
someone2: – Outrageous!
Marx: We have a Russian among us. Pavel…
You might have a role in his country, Weitling.
It's the only place for vague alliances between vague prophets and disciples…
_{––– [German version of the crucial line: "Nur dort wachsen solche seltsamen Verbindungen zwischen konfusen Propheten und konfusen Anhängern"]}
But in our countries, in Germany, France or England, nothing can be done without a positive theory.
Grün: A man who leads hundreds to justice cannot be treated like this!
Marx: A hypocrite like you should keep silent.

Source: The Young Karl Marx (2017) Movie Script at approximately 81min into movie.

In the English version of that movie the German word konfus is translated as "vague". Or the other way around. I do not know which language the original screenplay was written in. I will further abstain from judging that translation choice for correctness or appropriateness, not least because the origin and subsequently direction remains unclear to me. The movie is credited as being written by Pascal Bonitzer, a French, and Raoul Peck, a Haitian.

That line as alluded to in the question is not in the movie in this form. The closest lines in the movie that somehow seem to relate to the "quote" in question are clearly said by Marx and not by Engels.

Within the movie it also quite clear that only Weitling himself and his followers are addressed. "The Russians" are only called the one place/people where this vagueness might actually work! Ironically the movie is very vague itself, only using "Russian […] Pavel […] his country". As uttered in the movie the line is quite all-encompassing and might be interpreted to speak at and about almost anything and anyone. It's a general description and not really directed at a specific ideological entity, organisation, or person, or group. In this case, it is more of an artistic allusion to later developments that in hindsight sounds like a prophecy addressing the Lenin-type.

The main targets of attacks by Marx, usually

As the movie is about the young Marx, it seems a bit questionable that a quote like that was generated by the protagonist at that time:

Among the Russians who kept in touch with Marx and Engels there were men and women belonging to three generations of revolutionaries. In the 1840s the revolutionary movement in Russia had an almost exclusively intellectual and liberal character. It was based on no social class or popular force. To that epoch belonged Marx’s early correspondents, Annenkov, Sazonov and a few others. Marx explained to them his philosophy and his economic ideas, but engaged in no discussion on revolution in Russia. For this it was too early. Broadly speaking, in those years Russia was to Marx still identical with Tsardom, and Tsardom was the hated ‘gendarme of European reaction’. His and Engels’ main preoccupation was to arouse Europe against that gendarme, for they believed that a European war against Russia would hasten the progress of the West towards socialism.
Isaac Deutscher, 1948: "Marx and Russia"

As can be seen in the letter to that Pavel Annenkov in the movie, linked to in the above quote, Marx is primarily concerned with attacking Proudhon:

Mr Proudhon confuses ideas and things. Man never renounces what he has gained, but this does not mean that he never renounces the form of society in which he has acquired certain productive forces. On the contrary. If he is not to be deprived of the results obtained or to forfeit the fruits of civilisation, man is compelled to change all his traditional social forms as soon as the mode of commerce ceases to correspond to the productive forces acquired.

Digging deeper: attributing the movie line?

Interestingly, in the movie it is Weitling who is attacked. And Weitling wrote about that incident in a letter to Moses Hess:

Last evening we met again inpleno. Marx brought with him a man whom he presented to us as a Russian [Annenkov], and who never said a word throughout the whole evening. The question was: What is the best way to carry on propaganda in Germany?

_{Letter by Wilhelm Weitling to Moses Hess, Brussels, March 31, 1846}

Unfortunately, Weitling does not mention anything about Russia or Russians, except that Annenkov was present and did not say a word. Not saying anything can make a good listener!

The surprising source for the movie scene

Let us now take up our second source. Its interest is enormous, for it is the proof that for Marx and Engels the break with Weitling was dictated by a deep concern for the entire future of culture, that is, by eminently humanist motives. It is a strange piece of good fortune that it was the same Russian whom Weitling mentioned as Marx' guest who is our witness for the meeting of March 30, 1846 and gives us a detailed account free of any political or national bias. He was the bourgeois publicist, Paul Annenkov (1812-87), whom a mutual friend, the "land-owner of the steppes" G. M. Tolstoi, had recommended to Marx. […]

But here is Annenkov's description of the meeting, word for word:

He would no doubt have spoken even longer if Marx, with eyebrows raised in anger, had not interrupted him to reply. The essence of this sarcastic response was that it was nothing but deceit to rouse the people without giving them a solid basis for their action.

By awakening the fantastic hopes just spoken of, Marx continued, one will never save those who suffer, but one will certainly lead them to their ruin. In Germany especially, going to the workers without scientifically precise ideas and without concrete teachings is the equivalent of making dishonest propaganda, without knowing what one is doing. That requires, to be sure, an enthusiastic apostle on one side, but also on the other nothing but donkeys who listen with their mouths open. Here, he added with a brusque movement of the hand, here we have a Russian among us. In his country, perhaps, the role you are playing would not be out of place. There and there only unions of absurd apostles with absurd disciples can form and subsist, with any real success.

In a civilized country like Germany, Marx continued, nothing can be produced without and concrete doctrine; and up to now nothing has been prod noise, a harmful excitement, and the ruin of the very cause on his hand to.
Weitling's pale cheeks colored and his speech suddenly became free and lively. In a voice trembling with excitement, he undertook to prove that a man who had gathered hundreds of men around him in the name of the idea of justice, solidarity and fraternal love, could not be called a trivial and useless man. He said that he, Weitling, was consoled for the attacks of that evening by the hundreds of letters, declarations and testimonials of gratitude he received from the most distant provinces of his country;, he said that his modest preparatory activity was perhaps of more importance for the common cause than the library criticism and analysis deployed apart from the suffering world and the people's torments.
As he pronounced these last words, Marx, furious, smashed his fist the table so violently that the lamp swung. Then rising, he cried: "Ignorance never did any one any good."
We rose too, following his example. The meeting was at an en And while Marx walked from one side of the room to another, in an unfamiliar agitation and rage, I rapidly took leave of him and the others and went home, astonished to the greatest degree at what I had just heard and seen.
"Ignorance never did any one any good!"
This is the true death sentence on all the primitive equalitarian communism, which good old Buonarroti thought should in Justice's name have put the humble shepherd and the scientist Newton on the same footing. It was the death sentence on all the levelling utopianism of the masses, intrinsically anti-cultural, which had dominated the entire epoch.
_{Hans Mühlestein: "Marx and the Utopian Wilhelm Weitling", Science & Society, Vol. 12, No. 1, A Centenary of Marxism (Winter, 1948), pp. 113- 129.}

Translation is hell on earth. A Russian reports from a night in Belgium what a German said in either German or 'horrible French' which is then written down in Russian, quoted here in an English translation and then compared to a movie written by two French native speakers in its German and English soundtrack versions.

Conclusion part 1:

Within these just enumerated caveats:

• Movie English: It's the only place for vague alliances between vague prophets and disciples…
• Movie German: Nur dort wachsen solche seltsamen Verbindungen zwischen konfusen Propheten und konfusen Anhängern
• Annenkov/Marx: There and there only unions of absurd apostles with absurd disciples can form and subsist, with any real success

Despite some obvious liberties the movies takes and concerning language has to take: It is not in or from within the written works of Marx. It is not an indisputable quote. But as an attributed quote it is as close as it gets to a real event as reported by a witness some years later.

Part 2: Main groups or persons from Russia Marx was critical about:

Since this is spoken word in a dramatised bio-pic, there cannot be any guarantees on historical accuracy. In fact, the opposite is likely true and we can quite safely assume that these words were never spoken in that precise way from the start. But in this case, as shown above, it comes quite close to the real history as it presented to us.

But assuming that this is generally a movie trying to be honest about its source material:

But if there is a source on which this statement about the probability of either Weitling's success or the appropriateness of vagueness, konfuse or absurd in Russia: then it is presumably not about any communists, Russian or not. It is presumably also not about socialists. Apart from the addressed Weitling in the movie: The most likely candidate for this kind of attack in a colourful pastiche of sentiments that are in the works of Marx would be in its early phases Proudhon and in later times the anarchists around Bakunin, especially if a focus is indeed put on Russian circumstances and situations:

They are told that the truth, too dazzling for eyes that are still unfamiliar to true anarchism, is only fully revealed in the program of the Russian section. Only to the born anarchists, to the chosen people, to his youth of holy Russia the prophet dares to speak openly. There anarchy becomes the general destruction of all, revolution becomes a series of first individual, then mass murders; the only rule of conduct is the increased Jesuit morality; the archetype of the revolutionary is the robber. Young people are banned from thinking and science as secular activities that could lead them to doubt the all-destroying orthodoxy. Anyone who persists in persisting in their theoretical heresies or in applying the standard of ordinary criticism to general amorphism is threatened with the Holy Inquisition. Before the Russian youth, the Pope no longer needs to be compelled, neither in content nor in form.[…]
[Footnote 7 on that page:]
We must note here that in the Russian language the words association, union, alliance (obschestvo, sojuz, tovarischestvo) are more or less synonymous and are used without distinction. The word: International is also usually translated as general (vsemirnyi). The Russian press thus often translates the "International Association" with words that could just as well mean the "general or universal alliance". By using this linguistic confusion, Bakunin and Netjaev managed to exploit the name of our association and bring misfortune to almost a hundred young people.
_{Karl Marx/Friedrich Engels - Werke. (Karl) Dietz Verlag, Berlin. Band 18, 5. Auflage 1973, unveränderter Nachdruck der 1. Auflage 1962, Berlin/DDR. S. 396-438. "Die Allianz in Rußland"}

But it seems to fit a lot more passages as well, if one is willing to grant a certain artistic licence and liberty for quote hunting. Original Marx is an often a great read, highly entertaining especially in his letters, but also an audience attention killer on screen.

Not always did he manage to condense a bonmot in so few words like this:

"Tout ce que je sais, c'est que je ne suis pas Marxiste."
("All I know is that I am not a Marxist.")

Although this good one leaves open the possibility –– and I do that as well explicitly here for my two cents as well –– that any such attack might be directed at everyone², including Russians that were supposedly on his side/following him.

But Bakunin was on every shortlist of 'Who should be criticized next?':

The second decision on the political attitude of the working class put an end to the confusion that Bakunin had wanted to bring to the international by including in its program the doctrine of full abstention in political fields. - The third resolution {1} threatened Bakunin directly. Further down, when we talk about Russia, we will see how interested Bakunin himself was in hiding the vices of the Alliance from Western Europe.
_{Karl Marx/Friedrich Engels - Werke. (Karl) Dietz Verlag, Berlin. Band 18, 5. Auflage 1973, unveränderter Nachdruck der 1. Auflage 1962, Berlin/DDR. S. 347-362. "Die Allianz in der Schweiz"}

To explain this accusation of "confusion" in relation to Bakunin very succinctly:

Hence, from this perspective society appears to be as inflexible as the law of gravity. But the goal of a socialist society is to invert this relation. Instead of individuals feeling powerless in the face of their own social institutions, by directly coming together through organized discourse, they place themselves in a position to alter these institutions according to their own needs and values. But this can only be accomplished when individuals are operating as a coordinated force, where they are discussing, debating and voting on which options to pursue, and where everyone has the opportunity to participate. Consequently a socialist society brings into play a new definition of freedom, and, in Marx’s opinion, a superior conception: the collective, rational determination of social policy.

Freedom in this field can only consist in socialized man, the associated producers, rationally regulating their interchange with Nature, bringing it under their common control, instead of being ruled by it as by blind forces of Nature. (Marx, Karl, Capital, Volume 3, p.820.)

Consequently, Bakunin’s individualistic definition of freedom, in Marx’s opinion, remains mired in the conceptual framework of bourgeois philosophy and simply sows confusion when transplanted onto a socialist foundation:

Liberty [i.e. the bourgeois conception], therefore is the right to do everything that harms no one else. The limits within which anyone can act without harming someone else are defined by law, just as the boundary between two fields is determined by a boundary post. It is a question of the liberty of man as an isolated monad, withdrawn into himself.... But the [bourgeois] right of man to liberty is based not on the association of man with man, but on the separation of man from man. It is the right of this separation, the right of the restricted individual, withdrawn into himself. _{(Marx, Karl, ‘On the Jewish Question’, Collected Works, Volume 1 (New York, 1975), pp.162-63.)}

We can now see that when Marxists and anarchists refer to such concepts as “human nature” and “freedom”, they have diametrically opposed definitions in mind and therefore are frequently talking at cross-purposes. Bakunin’s notion of spontaneity stands starkly opposed to Marx’s notion of collective, rational action. Each author, armed with his own definition, could then logically categorize the other as a tyrant. One can understand, therefore, why Bakunin labeled Marx an “authoritarian” when Marx would not concede to Bakunin’s impulsive politics. Marx, on the other hand, viewed Bakunin’s conceptual framework as mired in an antiquated 18th century Enlightenment philosophy, lacking any historical dimension, theoretically inconsistent, and parading metaphysics as if it were materialism.
_{Ann Robertson: "The philosophical roots of the Marx-Bakunin conflict", libcom.org, 2017}

Conclusion part 2:

The conclusion is that despite some reasonable guesses this has to remain inconclusive. The most revealing point is that compared to Russian or other Marxists, Karl distanced himself from being a Marxist. The most important factor here being that with all the sympathy he regularly expressed towards revolutionary tendencies and actions taken in Russia, he firmly believed that – as detailed in his theory of scientific socialism – the circumstances in Russia would make an attempt at these stages of development unfruitful, dishonest and or dangerous. Just to stop enumerating at three. There were more.
For an additional reason I refer to Pieter Geerkens answer. Intellectually and on a philosophical or theoretical level the revolutionary most attacked by Marx would be Bakunin still. Almost just coincidently a Russian.

(1) Since the movie also takes liberty with sources, I treat Marx and Engels here as the team of writers they were and attribute the writings of Marx as edited together by Engels as created by one author.
(2) Since in the context of the movie Weitling is adddressed with this speech, it is interesting to note that among his disciples there was a "genuine prophet" of communism called Georg Kuhlmann. Almost needless to say that he was an agent provocateur in Austrian pay?
_{Kuhlmann, Georg: Scharlatan, der sich als Prophet ausgab und unter den deutschen Handwerkern und Anhängern Weitlings in der Schweiz in religiösen Phrasen den „wahren" Sozialismus predigte; erwies sich später als Provokateur im Dienste der österreichischen Regierung. (Charlatan, who posed as a prophet and preached "true" socialism in religious phrases among Weitling's German speaking craftsmen and followers in Switzerland; later proved to be a provocateur in the service of the Austrian government) Source: Engels and Marx, MEW, Band 30,435}
(†) This refers to the original version of the question title: "Is this a true quote of Engels about confused Russian leaders and their confused followers?"

I cannot certify that the precise wording is accurate, but the sentiment most certainly is. This essay on the relationship of Marx and Engels with the 19th Century Russian revolutionaries notes this statement by Engels (my emphasis):

Consequently, the existence of the bourgeoisie is from this point of view also as necessary a condition for the Socialist revolution as the proletariat. A person who maintains that this revolution could be carried out more easily in his country because it neither has proletariat nor bourgeoisie, proves by his statement that he has understood nothing of Socialism.

The Russian revolutionaries were constantly asserting that Russia was a special case, where the pre-existence of bourgeoisie and proletariat was not a prerequisite for a successful socialist revolution.