That's an unsubstantiated claim. I don't think it's true. From what I've heard about the YC interview and decision-making process I think they place a high value on maturity. The ones I've met have fun but don't have rockstar attitudes.
Also the awkwardness she's talking about doesn't come only from having a big ego. It's from constantly saying a single asinine statement. The person saying it often doesn't mean for it to have the effect it does.
"Rockstar" is a meaningless word. Real rock stars, like famous musicians, work their butts off with large teams, and then celebrate, sometimes too hard.
Also the awkwardness she's talking about doesn't come only from having a big ego. It's from constantly saying a single asinine statement. The person saying it often doesn't mean for it to have the effect it does.