Dumbledore behaves very strangely in this chapter.

He likens Riddle's spirit to a dumb animal, which does not know that it was sent away. That's a sad, sympathetic image.

He laughs at the skewed symmetry between Good Riddle and Evil Riddle, saying that this is what Riddle could have been if he'd been raised by parents who loved him. If you feel any sympathy for Riddle at all then that's not funny, it's tragic: Riddle's crimes and suffering, his whole live, arose from sheer bad luck on his part. To think it a joke, or to expect Riddle to share it, is something I don't understand at all.

EDIT: Also, he's not taking the prophecy very seriously anymore, did you notice? "Oh well, I trapped you in time so Harry will defeat some other dark lord". One wonders what Dumbledore's "unusual power of Divination" is.

But I don't think that necessarily means he's a fake, inadvertently conjured up by Riddle (one wonders why real!Dumbledore would throw away the Wand and Line, for example, but why would fake!Dumbledore do so?). I don't know what it means.