I understand the difference and don't pretend to know the daily goings on at YC, am just putting an alternate perspective on the table for consideration. Tonight when you reflect on this conversation, I think an idea will come to you. I am sure this is something that bothers you very much, because you are obsessed with solving tough problems. I think people also forget that you are human and have feelings and your not always on the top of your game. When people are overcome with a tough process they shut off. As you stated, that doesn't mean they don't know how or that they are not capable. They just don't know how to get back on the right track. These situations don't need force they need delicate leadership. If they don't respond from there, then you have done all you can and that is all anyone could ask for.
I even up voted you because I know you have taken time to have this conversation. I most likely will never get into YC because I am a single founder, therefore I fear no recourse. When the terms are level, inspiring conversation and progress can be made. I appreciate that you have listened to me and considered my thoughts, as someone trying to reach a goal, that means very much!
Think about this conversation and then what I just said, the answer is right there staring at you my friend.
That of course is no reason for pg to not try to relentlessly improve, but the point about the founders stands.