That would have been a great follow-up question.
The application must have had a description of an idea that was good enough to get the interview. Probably the interview starts by asking the team to explain their plans in more detail. If after 5 minutes pg & friends don't have a clearer picture than what they got from the application, then is the team going to be able to explain the idea to customers (who are not going to give them 5 whole minutes)? Or be ready to pitch VCs in a few months (which is even more stressful)?
It does not seem possible that an application where the only description is "we're improving the way people interact" gets an invitation; so, when the interview is a little shallow (because of stress, etc.) why not start back from the application?
Or maybe the interview is not about the idea, but about the ability of the founders to communicate effectively...?