Yes this is correct. If you have the event metagross in your party, at several points dialog will change . His dad remarks about steven having a beldum colored like yours, and would love to see yours fight it. The fossil manic also tells you that steven went all the way to kalos to find it and the stone. Later in the delta episode, he will add additional dialog that talks about he and that metagross met rayquazza once and then begins to wonder what happened to Allan. ( he says that he wonders where that man and his black charziard went.) Then finally, in the letter attached to the normal beldum he gives you. He tells you he also sent a different colored one and hopes it got to you. He tells you he sent both to get you to love steel types. This was all hinted to happen beforehand though when he says in the demo one day he will give you this metagross. So this confirms that at least the mega evolution act is cannon.In the Special Demo Version of OR/AS: Steven said that as a present, he will give [Player] the odd colored Pokemon (Shiny Metagross) sometime in the future (which is currently one of the active Mystery Gifts found in the game). That explains why he's using a regular one in-game.