The amount of capital provided by YC is trivial...a rounding error relative to the value of a successful company. But more relevant is that YC almost certainly has some meaningful data on the companies it rejects.
So PG's claim could very well be supported by proprietary information.
Trivial once the company is successful, but isn't the purpose of the money to ensure that the founders don't have to give up and get "proper" jobs before the company succeeds?
So PG's claim could very well be supported by proprietary information.