The definition of legality in Pokémon hacking is your definition of Legitimacy. An illegal Pokémon is not recognized as an legitimate Pokémon. Azure Flute Arceus is only recognized as a legal (or in your case, legit) Pokémon in 5 Games, those being HesrtGold SoulSilver Pearl Diamond and Platnium. However, Nintendo has has confirmed themself that this Pokémon should not exist. Let me put it this way. It can’t be transferred, it’s not legal (legit).