>>53622822>How is there a Mega Stone made for a Pokemon that would not exist for 3000 years?Because they didn't just suddenly make all the mega stones 3,000 years ago. We literally get this explanation in ORAS with Infinity Energy and get to witness the creation of new mega stones after the climax with primal Pokemon.
Stop assuming your favorite Youtuber's headcanon is canon. There's no such thing as a "mega" or "non-mega" universe either. Play the games.