No, because they retcon this every time a new game comes out. Why didn't we see the Pokemon from future expanded dexes in DP and BW? Why didn't Lickitung and co evolve before DP? These sort of discrepancies are inevitable and I don't think they really need to explain them.No, like, if Alola Pokémon were in Kanto and adapting to the region, wouldn't we have seen this during the events of RBY and such?
Unless the multiverse plays into this.
If it's that big of a deal they can just be encountered in new areas or something.