... and he was 13. Which, yes, is a very bad thing, but unless the company investigated that claim (e.g., to assess potential PR fallout) and there was some significant deception by Altman against the board in the context of that investigation, its not something that would get him fired with the explanation OpenAI has provided.
(OTOH, the accusation and its potential PR impact could be a factor that weighed into how the board handled an unrelated problem with Altman—it certainly isn't helpful to him.)
I'm not saying this happened or it didn't. But just that it could absolutely be more than enough to fire anyone.
I don't disagree that the accusation alone (especially if it stood up to modest scrutiny, and looked to be ongoing PR issue, even if not well substantiated enough to have confidence that it was likely to be true) might be sufficient for firing; CEOs are the public and and internal face of the firm, and so PR or employee safety concerns that attach to them are important to the firm. But it wouldn't be for lack of candor with the board unless there was something for which the board had a very strong reason to believe Altman was dishonest in a significant way.
They could easily fire him with the lack of confidence language without the lack of candor language.