YC on the other hand was clearly informed that there was an individual who was considered a founder at some point in time. Despite knowledge of this risk, they invested and they facilitated follow-on financing. Now there's a big, dumb, expensive problem. YC dropped the ball, big-time.
The fact that Sam Altman is turning this into a public spectacle is further evidence that he knows YC fucked up badly, and probably doesn't have a legal case. The only reason to write what he wrote is if you're out of actual ammo.