I have a few corrections to point out on that theory:
-Volcanion was never mentioned in the
leaks, hell, the blackouts weren't even mentioned from what I can find. I don't know where he got that information, but that was not in the Pokebeach summary.
-Volcanion is never connected to the Power Plants at all, the datamined data from XY and ORAS (which he does talk about) never shows that its supposed to be there and dialogue about it doesn't say anything relating to it. So its connection to Team Flare and their actions are flimsy at best.
-Minor nitpick, but Zygarde is heavily implied to
not have a large role in the movie. Granted, this theory probably predates the trailer, but the trailer
here has it show up in the same manner as the other Kalos legendaries that had movies while Volcanion was the focus, so it's likely not going to be a major part of the movie.
-Team Rocket creating Mewtwo was an anime only plot point, so that can't be a Kanto parallel.
I personally also think that they won't involve a recently released Event Pokémon into the plot of the upcoming game (especially since it's supposed to released for ORAS), but that's my opinion on the matter.