I think that's only true for people who've never been humbled. If you've been humbled, you are more easily able to question yourself and recognize if you're not the smartest in the room. At least, that's the way it worked for me. :)
I think it mostly has to do with whether someone's ego is all wrapped up in his or her intelligence, which is pretty common for people of above average intelligence. If it is, judgment of relative intelligences will probably be clouded, at least as it relates to the person, since evidence that others are intellectually superior in a particular area will often be treated as a threat to the person's identity and ignored or rejected, often subconsciously.
If the person has a strong feeling of self worth that is independent of intelligence, they won't care very much, and so can be more objective about whether they're the smartest person in a room, regardless of if they are or aren't.
Put another way, there will undoubtedly be situations for a very high percentage of human beings where they happen to be the smartest person in the room. Being able to recognize this doesn't necessarily indicate arrogance or an inflated perception of one's intelligence.
Of course this issue is also obscured by the entirely vague and amorphous nature of the very concept of intelligence. There likely exist countless 'mentally retarded' people who have much higher emotional intelligence than a typical MIT genius, and are thereby probably better equipped for a happy, successful, and socially beneficial life.
I don't think someone needs to 'be humbled' to get it. They just need to be humble. It -is- possible to learn lessons without having your nose ground in it.
Also, there's a difference between thinking you're smartest and knowing it. Mostly in attitude.
