The database is not incorrect for excluding encounters that only occur through glitches.
Worth mentioning here: Mew was completely inaccessible in the original Ruby and Sapphire until it was distributed in a event, almost three years after the games originally released. So in a way, yes you're right, Mew didn't exist. It's not a prerequisite that you can encounter Mew in every game. That rings true for X as well.