Again, I feel portals being the major evidence for alternate universes seems unreasonable. It is very clear that the earlier Mario games intended the Mushroom Kingdom, Sarasaland, etc to be other locations on Earth, since a lot of arcade games were implied to be in New York. Obviously, this was retconned to New Donk, though things like Mario and Luigi living in Brooklyn for some of their life, and there being games set on actual Earth, it seems Nintendo is implying two separate worlds in one universe, connected by a portal. At the very least, the Mushroom World is not meant to be its own version of Earth - that'd be like saying Legend of Zelda takes place on an alternate Earth because there are humans