This makes no sense, but if you think about it, neither do Dumbledore or Inception. Dumbledore says lots of things that make sense in the world of Inception, and this is some good advice for Leo.
Of course, Dumbledore was probably the first person to complete a successful Inception when he planted the idea in Leo's head that "Of course it is happening inside your head, but why on earth should that mean that it is not real?"
Damn, you Dumbledear, planting all sorts of ideas that cause problems. Now he has to go back and fix it all so Leo can go back to his kids, or whatever.
This is so stupid. The movie, however, was not. It was great.