I like the part where he speaks about how one should be one's own surrogate customer. It is very difficult to be on the same track when starting a startup. Only if it is pain point for you, you will try to find the right solution.
So the idea of focusing on "being your own surrogate customer" is a way of breaking out from the rigid mindset of "either being a solution looking for a problem" which is what engineers do or "a problem looking for a solution".
By being a surrogate customer, I am talking about the latter but with the additional emphasis that it is a problem we ourselves experience.
This is particularly important for the YC class since their advantage is not just that they are "hackers" but that they are also representatives of their own generation.