If they're not even coming out and saying it's a new Mewtwo form, then something fishy's going on. Oh well, you know the Genwunners will have a field day regardless - even with the prospect of a 90s schoolyard rumor possibly coming to life. (First Luigi in Mario 64, then thawing out Zora's Domain, then the General Leo glitch, now this. What next, Aeris comes back and says it was just a flesh wound?)
Edit: There's a huge catch now... it's being advertised as a new Pokémon IN ENGLISH. I think they enhanced the truth in that way in Japan before, but between their intent on keeping its identity under wraps, as well as this, it may be a new species entirely, despite looking like the textbook definition of an alternate form.