Doctor Who "canon" has never been a rigid, consistent thing. New stories have either acknowledged, contradicted, or ignored what's been done in the past, depending on what best suited the needs of the moment or the inclinations of a given showrunner.
And even as far as a strictly literal reading of canon is concerned, the Curator never explicitly confirmed he was a future Doctor. He just said "might" and "perhaps."
It's a complete misreading of the scene to take that as definitive, unarguable confirmation that the Curator absolutely is a future Doctor. That's not what the Curator said, and it's not what Moffat meant him to say. His speech was deliberately left as ambiguous and open to interpretation as possible, because it wasn't meant to be some deep revelation about canon, it was just an in-joke for fun.