The opener is such a load of nonsense it's hard to know where to begin.

Why? Never in the discussion did the author claim that "Hitler was a creationist, therefore atheism is a valid concept (or creationism is not a valid concept)." The point was to show that there is more than enough evidence to contradict this common, and seemingly effective, anti-atheist argument that links Hitler to atheism, darwinism, etc. And if you are correct that Hitler just switched around to whatever view was most helpful to his political goals, then that undermines the religious propaganda about atheism and evolution being integral to Nazism and Hitler's ideology as well.

You're absolutely right that Hitler's beliefs are irrelevant to the legitimacy of atheism. That is true even it could be shown that Hitler genuinely tried to ground his beliefs in atheism and evolution.

However, very few people understand the difference between correlation and causation. If they did, these arguments about "what Hitler believed" would immediately be dismissed by "so what?". But the fact is, we have to deal with a popular perception that Hitler was an atheist, and that atheism has something to do with his behavior. Producing evidence that directly contradicts the first premise cuts to the heart of this myth in a manner that is probably more effective than merely pointing out the logical fallacy of the argument.