When Arceus first were leaked, fans seemingly were confused it, because it seemingly got the role as a god Mew had in the three earlier generations - especially as that wasn't retconned.
After having read a bit about both however, I've noticed that it's just a BIG missunderstand amongst fans, that kind of missunderstand nobody realises. Kinda.
The thing is, the game, which is the MAIN CANON, have just mentioned Mew being the ancestor of all Pokémon, while Arceus created the universe. It's never been claimed that it was Mew who created the universe, neither that Arceus created all the Pokémon (except the dragon and lake trios, of course).