I would much rather have a QB that believes in himself than a QB that thinks he's OK or pretty good or above average. I personally would be more worried about a QB that says "Yeah, I'm not bad I guess..." and gives you the old shoulder shrug. Would it have been easier for some to swallow if he said top 3? Top 5?
As I've said before, there's guys out there that don't want the rock with the game on the line. Give me someone who does. Manning is one of those guys who does. Is he the best? Debatable, but IMO he is right now.