Another anecdotal data from an arxiv preprint [0]: "After the paper was written, the author learned that the same result was also proved independently and by a different method by Ningyi Li [7]. The result is also contained in a paper [8] released by OpenAI on October 6, 2026, in which the proof strategy and specific choices of notation are identical to a preliminary version of the present paper that was uploaded to ChatGPT on September 8, 2026."
So not only did OpenAI publish a proof similar to one discussed on ChatGPT, but also there were other possible strategies to prove the result, that OpenAI didn't follow.
Also, up to my knowledge, on the free plan, OpenAI doesn't claim that they won't use the chat data for training. So this seems plausible.