I've been hearing that rumor since gen 4 when I got into pokemon. This gen it does look much more accurate than past gens, there is some suggestive evidence of the psychic/fire type (suggestive evidence being several psychic type attacks being shown very often in trailers and demos), same for the the grass/dark (dark type moves have been shown a lot, and in the anime trailer he suddenly closes his eyes and clenches his fists as if he's having a bad thought of some sort), none whatsoever of water/fighting except that some people thought he used a fighting type move in the trailer, but it looks nothing like any existing fighting type moves and looks nearly exactly like Acrobatics, a flying type move. He also knows bounce and has a supporting pokedex entry for flying type.
In case you haven't noticed, I'm incredibly biased on Froakie.
There may well be a new fighting type move that happens to look exactly like Acrobatics, and several fighting types learn bounce. So there's not really a 100% accurate yes or no yet (as for as I know at least)
It's really up to you to decide until the games come out, but for now I'm gonna call this one another fake.
The game seems to be based on good vs. evil (just look at the legend's names for crying out loud!)
maybe the X legend is trying to mega evolve to take out evil, while the Y legend is trying to mega evolve to take control of... Whatever a giant, evil, flying letter of the alphabet would want to take control of. Soup maybe.
And something completely off topic, I find it Hilarius that all these years they've been saying there are no bad pokemon, yet they're making one that's name has "evil" in it.