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Champ has had the mega stone on her since we first saw her, so its probably implied that she at one time trained there. I think there may have been a mention by some NPC that Diantha was once there but I dont remember exactly.
Your rival was explicitly stated to have gone there to train, so it makes sense she would have figured out how to mega evolve.
now, how they got a device just like yours? Well that's the mystery. Where did Diantha get her necklace from? Where did Calem/Serena get the other Omnitrix if there was only the one that you got?
