Having interviewed many people over the years (not for an accelerator though), you can easily tell within ten minutes if someone can not pass muster. I have never had anyone who stumbled over the first three questions I asked them, recover on subsequent questions. Nowadays, the only reason I keep interviewing people who stumble over the first three questions is out of politeness.

As I am usually interviewing people for one position, which they need to perform well in, it would take more than ten minutes to get to a yes. But since YC gives sixty or more yeses a batch, and they still win if only a few of them pan out to be a Dropbox or Reddit, than they can probably afford to give a yes in the first ten minutes as well. Because who is a superstar is obvious in the first ten minutes as well. The only question for a superstar beyond the first ten minutes is if they are a fit or not (for example, would the obviously motivated and skilled author of Temple OS, who is marked dead here on HN, be a fit).