I disagree with you here. There are far more universes at play than only those created due to time travel hijinx. Throughout the Hickman run, characters have repeatedly referred to infinite universes. The various What If? series further establish that the Marvel Multiverse assumes some sort of variation of the infinite multiverse theme.
I think that we have to make that assumption. I can go with it, because it's a comic book story, but such stretches are part of the reason this explanation as it has been presented so far has been unsatisfying.I would hazard a guess that either all alternative universes have an Earth or an analogue Earth, or universes without even an analogue Earth still have a Molecule Man