No, Myriah is not legit. Just because someone else has that name doesn't mean it has a history as a name. It can commonly be seen on baby naming sites that list any version of any name that was ever concocted. They may even make up a random meaning for the name. Really, it is just a neologism. It is just another recreation of the name much like people will misspell
Melanie as Melluhnee.