Is it true that Germans in World War 2 did not suffer punishment for refusing genocidal orders?

Yes, it is true. To the extent that one can prove a negative, we know that Germans were not killed if they disobeyed orders to kill Jewish civilians.

See this exchange between Daniel J. Goldhagen and Christopher R. Browning. In his presentation on his book Hitler's Willing Executioners, Goldhagen states (per p. 17 of the PDF):

The perpetrators were not coerced to kill. Never in the history of the Holocaust was a German ever killed, sent to concentration camp, jailed, or punished in any serious way for refusing to kill Jews. It never happened. Moreover, in many units officers announced to their men that they did not have to kill, and in at least nine police battalions the men had been informed that they did not have to kill. There is similar evidence for the some of the Einsatzkommandos. There is also evidence that Himmler himself issue orders allowing those who were not up to the killing to be excused from it.

Browning, like his opponent, points out their disagreement on virtually every aspect of the subject which academics could disagree about. However, he goes out of his way to affirm their agreement on this point, and helpfully provides an example unrelated to reserve battalion 101 by quoting from Nechama Tec's book In the Lion’s Den (on p. 27 of the PDF):

A selected few Germans, three out of thirteen, consistently abstained from becoming a part of all anti-Jewish expeditions.... No one seemed to bother them. No one talked about their absences. It was as if they had a right to abstain.

Edit: re smell test - in history noses are good, but sources are better. Sometimes what is counter-intuitive and outrageous to our notions of human nature is also the truth.

The non-coercion of participants is one of the most important and least appreciated aspects of the holocaust, notwithstanding that it's well-known among academics. For that reason I'd like to address some of the comments in my answer.

No repercussions ever for refusing direct orders? This claim doesn't exactly pass the smell test, does it?

The quote in the question doesn't say that no-one ever faced repercussions for disobeying direct orders. It depends on what the orders were.

The quote in the question said

If they order police battalion to go shoot Jews, you didn't have to do it. You could pull out. They never killed anybody. They never executed anybody. They never even punished people for saying, "no I'm not going to do that." So people are doing it voluntarily.

In other words Germans specifically weren't punished for disobeying orders to murder Jewish civilians. They were executed or imprisoned for some other challenges to Nazi authority.

In the PDF I linked, Browning described some of the things which did get people killed.

Repression was real. Bishop Galen, by virtue of his visibility and status, barely
survived his condemnation of euthanasia. But students of the White Rose, who passed out leaflets
condemning the mass murders of the regime, were arrested, tortured, and beheaded. Members of the killing units could individually abstain from shooting, but those who encouraged others not to shoot were
courtmartialed for defeatism and subversion of morale.

As other comments pointed out, someone who refused to pull the trigger would just be replaced by someone who complied. It was quite a different thing to discourage others from obeying these orders.

a German who wanted to shoot at armed Soviets rather than unarmed Jews could usually find a posting to do just that at the Eastern Front.

This is fairly plausible for some potential refusers, but there are a few problems with it.

1. Many of the people who were told to participate in the holocaust were not up to military service - such as those police officers in Reserve Battalion 101. At the very end of the war the Germans sent practically everybody to the front, regardless of their degree of participation in the genocide.

2. If you want to say that it happened, you need to find an example of someone who was sent to the front as a consequence of refusal to kill civilians. No-one has done that so far.

3. Soldiers on the Eastern Front, including non-SS soldiers in the Wehrmacht, were some of the most likely people to be involved in killing Jews and other civilians. Since they were already at the front, fear of being sent there clearly can't have been their motive for participating.

Even if it was possible to suffer no official repercussions, like it was official policy, the men could not know what the consequences in war time of failing to participate along with their comrades -- I can think of two movies (Training Day and Casualties of War) where someone refusing to participate put his own life into jeopardy and this would naturally be a concern of anyone in a battalion who declined to do what everyone else was doing. I realize I am citing works of fiction but these movies showed what appear to be realistic behavior.

There is no doubt that Germans were under tremendous pressure to obey and conform. There was a general climate of fear which must have been influential. However, once you have seen or heard of someone who refused to kill and was not punished for it, you would have no reason to think that you would be punished for doing the same thing. According to Browning (Goldhagen disagrees with this one, FWIW), 'some 10-20% of the reserve policemen refused or evaded and
became “non-shooters”.' So it's not believable that a large number of perpetrators honestly thought they'd get killed for refusing to kill civilians.

This matter is entangled with one of the disputes within Holocaust studies, although it isn't the point of the dispute. That began with Christopher Browning's book Ordinary Men (1992), which is a detailed study of the German Reserve Police Battalion 101. Browning concludes from his detailed study that the middle-aged men who made up this unit mostly killed due to peer pressure and obedience to orders, but that they could get out of participating in massacres, although few did. To quote Wikipedia:

The commander of the unit once gave his men the choice of opting out if they found it too hard, and fewer than 12 men opted out in a battalion of 500. Browning provides evidence to support the notion that not all these men were hateful anti-Semites. He includes the testimony of men who say they begged to be released from this work and to be placed elsewhere. In one instance, two fathers claimed that they could not kill children and thus asked to be given other work. Browning also tells of a man who demanded his release, obtained it, and was promoted once he returned to Germany.

Daniel Goldhagen is another historian, who has written about Browning's work, studying the same unit, and claiming that Browning has misunderstood the motivations of its members. However, he agrees with Browning that members of that police battalion could avoid taking part in killings.

This does not demonstrate that all (para-)military personnel could refuse genocidal orders, but it does show that some of them did manage to avoid obeying them. In an armed service, outright refusing orders as a subordinate is usually totally illegal, but finding ways to avoid obeying them or to be exempted is far more practical. I suspect that Naimark may have over-emphasised his point, as is quite easy in unscripted speech. There is plenty of evidence that Nazis who were quite willing to execute thousands of people nonetheless experienced severe psychological damage from it. Quite a few went mad, or killed themselves.

The Nazis introduced gas chambers because they were trying to reduce the effects of the killings on their men. A unit commander, operating the ghastly parody of leadership that was common among Nazis, might well exempt men who were unwilling, so as to have them available for other tasks later. He might also consider the wisdom of pushing armed troops beyond their limits: he'll die if they shoot him.

Nazi Germany did not recognise any general right of conscientious objection. Members of Battalion 101 who opted out of massacres would still be expected to do all their other work and support the war in general. Anyone who refused to serve the Nazi state at all would have been treated like the Jehovah's Witnesses of Germany, half of who went to concentration camps between 1933 and 1945 after refusing to perform military service, join Nazi organizations, or give allegiance to the Hitler regime.

Things changed in the final weeks of the war. At that point, flying courts martial roamed German controlled areas, seeking out traces of defeatism and executing anyone they felt showed it. At that time, disobeying orders to massacre people would have been very dangerous, and desertion would be the best course of action.