If they could declare with certainty that Buckminster's and Alpoge's usage data had been totally excluded from training, would that set a worse precedent and reflect poorly on their de-identification process (and data access safeguards moreover)?

This may sound like a charitable interpretation of OpenAI's remark, but consider that the lie would be (I think) impossible to falsify from the outside. They could easily just say "no sir we didn't peek" unless:

1. The conspiracy to peek at codex sessions involved enough people that the risk of one snitching is non-negligible

2. Lawyers advised it would be a bad idea to make such a remark, whether true or false