I think it is a simpler reason. Basically it is a result of their narrative for the reason Israel exists:

1) West is overcome with remorse at not having saved the Jews in the second world war,
2) As such they “give” “Palestine” to Israel.

Now if it turns out Jews weren’t really killed then there is nothing to feel bad about and so you can “give” “Palestine” back.

NB none of the 2 pillars are true but this is the standard narrative.