The legal risk comes if Altman decides he wants a similar job and can't find it over the next few months or years, and has reason to believe that OpenAI's statements tainted his reputation.

OpenAI's board's press release could very easily be construed as "Sam Altman is not trustworthy as a CEO", which could lead to his reputation being sullied among other possible employers. He could argue that the board defamed his reputation and kept him from what was otherwise a very promising career in an unfathomably lucrative field.