> The idea that "25% of the companies are on a trajectory that could lead to a multi-billion dollar company" feels wildly optimistic to me
The key word in that sentence is "could", not "will". I'm pretty sure after all these years YC have enough experience and data to identify the "could"'s. But they're still just "could"'s. Doesn't seem "wildly" optimistic to me, just optimistic, based on previous track records.
Relevant xkcd: https://xkcd.com/605/
Strong early growth is a very positive sign. But growth is a funny thing and can be hard to sustain. The true market test may come later... from when you are trying to grow a $50 million dollar business into a $500 million dollar business.