You know, if a myth appears in a work of fiction, it is almost always true in-universe. If Sinnoh myths were not true, then the Dragon trio, the lake trio and Arceus should not even exist in the games. Also, the fact that in the games Dialga/Palkia were clearly seen creating a galaxy (or something like that) at Spear Pillar, AND (as someone else said) the fact that Arceus could create an entirely new dragon in HG/SS points towards Arceus being the creator of the Pokémon universe.
Also, I don't get why people think Mew created the Pokémon World. Mew was never meant to represent a deity, it was more like the idea of the origin of life on Earth and evolution. Just read its Pokédex entries (I know, the Pokédex is sometimes unreliable but it's better than speculation). Every one of them talks about Mew being a common ancestor and sharing its DNA with the other Pokémon. It is never stated that Mew created the world or something like that.
Just think about Arceus as God (christians, don't kill me D8) and Mew as the very first living thing on earth (like a bacteria or something).
I'm not a native english speaker, so I'm sorry if I make mistakes.