>he specifically notes that they used the paid subscriptions, iirc.

Where are you seeing that? He only says "We used several LLMs throughout: Anthropic’s Claude, OpenAI’s Codex, especially with GPT-5.6 Sol and, more recently, Astra. The latter was only used for writeups and auditing our arguments."

I might be missing something, but he doesn't seem to confirm that he opted out, at least in the written writeup, maybe he has on social media? He also likely had early access to Astra given the timing, and I thought early access customers couldn't opt out? (Am I wrong about that?)

It's in this statement he released (linked in the article) [0]:

"I pay for the tools my group uses out of my own research funds, including footing a large bill to OpenAI."

[0] https://cims.nyu.edu/~tristanb/statement.pdf