Besides for the above, I saw the following:
The gedolim did not tell people to not go to E”Y; they only warned them to not fall prey to Zionism if they did go.
It was very possible that the Nazis would have reached and attacked E”Y. So advising people to go there wouldn’t necessarily have been a wise idea, just from a political perspective.
Finally, who is to say that Hashem would not have allowed Hitler in to E”Y if there had been a mass emigration there. Do all the gedolim bashers also disbelieve in schar vaOnesh? Maybe that would have been Hashem’s will and justice, in that circumstance?