Didn't he say exactly that about "The Name Of The Doctor"? Yawn.
Moffat trots out similar quotes on a regular basis. "The Impossible Astronaut" two-parter was supposed to be game-changing. "A Good Man Goes to War" was supposed to be game-changing. "The Wedding of River Song" was supposed to be game-changing. To the fan audience, steeped in genre television and its conventions, they're not especially game-changing.
But to the Not-We audience that the series is pitched at, those episodes are
game-changing. The Doctor is killed, that's a big deal. Amy and Rory's daughter is River Song, that's a big deal. The Doctor wipes himself out of history, that's a big deal.
When Moffat talks "game-changing," he's not talking to us. He's talking to the audience that tunes in a few times a year and on Christmas Day. And for that audience, "The Day of the Doctor" will be a game-changing experience, whatever that experience is.