Lots of assumptions there though..
-Mew having DNA of all pokemon
remember that Arceus and co were not even discovered before Mew, and even if people knew of it, the scientists making this statement surely didnt go get a sample of Arceus DNA, so at its best it means all casual pokemon, but I interpret it as having the DNA of most of the first generation at least. Any more is speculation (well we could safely add babies and evos obviously..), but I wouldnt doubt it includes casual Hoenn/Sinnoh DNA...Unova is a much bigger question mark...
-It is ASSUMED to be the ancestor
Because it seems to have DNA of all pokemon. Its a theory. It might as well just be simply an extraordinary case, it might be extending its DNA everytime it comes in touch with more species, or some other explanation..
-Arceus is said to have created the universe
We cannot deny the factuality of this story unless they added more legendaries with similar backstories (like if the next game introduced a second Arceus-like pokemon). But we cannot confirm it either, since all we have is a myth.
But even assuming it was true, how the heck would anyone ever know? Its not like ancient people of Sinnoh would have been there to witness the birth of the universe, remember they didn't exist yet.
The observation that Arceus is a mere pokemon like any other (it is limited by the same things, has real stats, a type, can use 4 moves, can be caught and used in battles.......) is a very important one.
I mean, when you ask something like "if pokemon had a god..", (of course it all depends on definition of that word etc. but) how could possibly a pokemon be the answer?
How does being the assumed ancestor make it superior? Its more like jack of all trades, master of none...