To be specific it is another Hoenn, not just any random dimension like Ultra Megalopolis.

It's not just with Megas. It's a timeline split from the point of the firing of the ancient weapon ~3000y ago causing Megas and cascading into other infinity energy events like the Darkest Day and the forming of Area Zero.

Also to Occam's razor it, in RSE there is no meteor threat requiring Rayquaza to mega evolve. Groudon and Kyorge don't go primal. The Delta Episode doesn't happen. When speaking of a universe without mega evolution what other version of Hoenn could they be speaking of?

It serves as a timeline split that brings infinity energy into broader knowledge and speeds up technological knowledge as various entities investigate.

I'm not sure what else I could call it since mega evolution literally doesn't exist in gen 1/3 but does in the Let's Go remakes, doesn't in gen 3 but does in ORAS despite being the same regions going through the same crisis.

Also I think there was an interview where someone mentioned that basically every game cartridge/save file was a separate universe because literally it's always a different player behind the protagonist.

It's mentioned in the endgame of ORAS when Zinnia bursts into the space center

As a certified, Oh goodness, I'm so high up! I agree. The fear of heights is real.