I think what he meant to say is that YC accepts applications from companies at all stages. Once accepted to YC, all companies are given the exact same funding deal.
If that's the case, I think that limits YC to reasonably early stage (pre-series A) or near-death companies. It's perfectly rational to give 6% to YC of a very early company. It's almost surely plus-EV to do that.
A successful company about to close a series-B? Much, much harder to imagine how YC would be plus-EV at that point.
A successful company about to close a series-B? Much, much harder to imagine how YC would be plus-EV at that point.