"Well, it's certainly true that it's always possible to underestimate more, if one tries."

That's not exactly a good thing, though.

"Take care that those nightmares don't get the better of you, though. They might be harmless upon waking, but that doesn't mean they won't be unpleasant!"

Yes, he realizes that saying that is likely to cause her to put even more effort into the (over)confident act. That's really quite possibly why he said it.