OpenAI has said that their models were definitely not trained on any of Buckmaster's sessions after July 3rd (from https://archive.ph/75WcF); likely they found that's when he switched the "allow training" setting off.
It's also possible he shared drafts of the work with someone else, who asked chatgpt to explain it to them with training on. Tao seemed to know lots of details of the work before anything was published, though also worked on the problem in the past with big results so maybe just guessed.
Is there an alternative link without certificate issues?