We're supposed to think that he's a future incarnation. Whether he is or not depends on whether Moffat is pulling our leg. Correction, I don't think that even Moffat is quite sure at this point, but he did write it as a future Doctor, so that's what it is until somebody makes up their mind and writes the story about it. Which unfortunately means never.
I like your interpretation too, there's something alluring to it, but he did know too much for a curator. I tried to imagine his words being a regular guy who has some knowledge about the Doctor joking, but so far I don't seem to see it working. But maybe it's just me.