Does Nagarjuna's Middle Treatise 24:18 teach real knowledge?

Nagarjuna is the master who gave a clear definition to the school of Buddhism that is known today as Mahayana Buddhism. So when you question whether what he thought in the quoted text is the real knowledge or not, it brought a smile to my face. How "emptiness" is defined in the Mahayana, and in the Pali Canon is different. I cannot remember where I came across it, so I will not comment further on this. You are not sure whether you have fallen into a trap or not.

All what I can tell you is that the Buddha taught us to be vigilant in the present and strive to understand the Dhamma by analyzing and investigating carefully. In society there are two predominant views. They are right view and wrong view. Right view leads to the proper path while the latter to a wrong road. One thing that I know is that both the Mahayanins and the Theravadins believe in the Four Noble Truths, the Eightfold Noble Path, the Dependent Origination, the Three Characteristics of Existence and a number of other basic tenets. Above all, both the schools accept Gotama Buddha as their Teacher. So that is a good place to start.

Some philosophical interpretations of Nagarajuna do phrase it in terms of dependently arising things not really being caused.

there is no causation, when causation is thought of as involving causal activity. Nonetheless, he [Nagarjuna] notes (1: 2), there are conditions... Suppose that you ask, "Why are the lights on?" I might reply as follows: (1) Because I flicked the switch. I have appealed to an efficient condition. Or (2) because the wires are in good working order, the bulbs haven't burned out, and the electricity is flowing. These are supporting conditions... [etc.] none of them makes reference to any causal powers or necessitation.

Garfield seems here to be saying that things are caused, but their causes aren't real.

I am quite unsure though whether he would say:

1. that's because knowledge of what causes what is deluded;
2. we can have non deluded knowledge

You are actually misunderstanding the verse.

The verse is in fact the KEY to unlocking the understanding of the Prajñāpāramitā sutras - Heart Sutra, Diamond Sutra as well the Mahāprajñāpāramitā Sūtra.

Here is an alternative English translation (You Zai) of the verse from Mulamadhyamaka Karika from the Chinese translation by Kumarajiva

All dharmas that arise from causes and conditions,

I call them emptiness.

They are also provisional names,

And also the Middle Way.

The Chinese text

眾因緣生法，我説即是空；

亦為是假名，亦是中道義。

What Nagarjuna is saying, is that because all dependent originated phenomenon are due to causes and conditions, by their very conditionality they are impermanent and without self. As stated by the Buddha.

Pratītyasamutpāda

"When this is, that is;

This arising, that arises;

When this is not, that is not;

This ceasing, that ceases."

Therefore because things are impermanent and without a true lasting self. He call them 'empty'.

However the term 'emptiness' is also just a convenient label, because it does not fully explain the complexity of the phenomenon and all the causes that made up something. To say something is 'empty' is that it ultimately lacks an underlying 'essence', but does not mean that it does not exist but that it does not truly exist.

This is therefore the Middle Way between Eternalism and Annihilationism. To be annihilationist is to say nothing exists and that there are no causes for anything to occur. This isn't the definition of emptiness as used by Nagarjuna. It is because there are causes, there are outcomes, but because they are made up of causes, there are also impermanent, hence is not Eternal.

So whenever you come across a text like the following:

Form is Emptiness, Emptiness is form.

Heart Sutra

Your should consider the meaning of emptiness.

Form is dependently generated, Dependent origination is form.

Form is impermanent, Impermanence is form.

Form is without self, Without self is form.

So the question would be, why did Mahayana Buddhist Nagarjuna rephrase what is already a known Buddhist concept into 'emptiness'.

It because of what they perceived as the reification of Buddhist teachings in the Abhidhamma by schools such as the Sarvastivada (one of the ancestors of the Theravada), recall that even in modern Theravada teaches that Nibanna is unconditioned, a deathless dimension, and that the Arahants are perfect etc.

Nagarjuna denies such permanence and eternalism.

This is why it is stated:

There is No Wisdom, and There is No Attainment Whatsoever.

Because wisdom is itself conditioned (by the practice of the Noble Eightfold Path), and therefore wisdom too is impermanent. Hence the fruit of Four Stages of Attainment (Stream Entry, Once Returner, One Returner, Arahantship) are impermanent too.

It is not saying that there wisdom does not exist or the fruits of the contemplative life does not exist. They do exist, but like everything else are impermanent and not self.

Nagarjuna is breaking our attachments to the attainments within the very Dharma itself! Even Nirvana itself is impermanent! The end of suffering as is suffering is caused and conditioned and is impermanent!

No, this is not an OK-ish understanding, since it does not approach traditional interpretations (whether that of Tsongkhapa or his opponents belonging to the Jonang school or the Nyingma tradition).

First, from the Madhyamika-Prasangika viewpoint (Buddhapalita, Candrakirti, Shantideva, Tsongkhapa, etc) there are thee types of dependent-arising:

1. Dependent on causes and conditions
2. Dependent on parts
3. Dependent on names

From the Madhyamika-Svatantrika viewpoint (Bhavaviveka, Haribhadra, Kamalasila, etc). There are only the first two types (or maybe this is forced on them by Prasangika proponents).

The point is: all objects of knowledge are dependent arising, even those that do not arise from causes and conditions and are therefore permanent phenomena (such as space, cessation, emptiness). This accords with the next verse of the same chapter:

Something that is not dependently arisen, Such a thing does not exist. Therefore a non-empty thing Does not exist.

Traditionally, we say that the meaning of dependent-arising is emptiness. This is an interpretation of the verse, since dependent-arising is not said to be emptiness, but the meaning of emptiness. This is because realizing emptiness does not equate realizing dependent-arising. One can not realize something as being a dependent-arising without having emptiness prior. That 'this is a provisional name' refers to emptiness being dependent-arising. Since it is a permanent phenomena, it does not depend on causes and conditions. However, it depends on parts and names: the emptiness of the mind, for instance, depends on its basis (i.e. the mind). Emptiness is necessarily emptiness of something and depends on its basis, therefore it is a dependent-arising.

This was stated because some think that emptiness exists truly (inherently) while it is not the case.

I suggest you attend teachings on this text and commentaries. One needs to rely on a teacher to study such texts. You started saying that you like simple statements, but it might be a trap leading you to think you can understand on your own.