So they get unlimited tokens and how is that limiting their access and usage exactly? Who's paying the bills with real money?

The unlimited tokens thing is a sign that YC are expecting all their startups to integrate AI as a core part of their product. It seems like a natural progression for AI to start purchasing things autonomously. My bet is that YC also think this and are building tech that can do this with stable coin - "AI shopping" regimes.

YC are presumably paying for the usage with fiat.