1) a confirmation of the dates of employment
2) a confirmation of the role/title during employment
3) whether or not they would rehire that person
... and that's it. The last one is a legally-sound way of saying that their time at the company left something to be desired, up to and including the point of them being terminated. It doesn't give them exposure under defamation because it's completely true, as the company is fully in-charge of that decision and can thus set the reality surrounding it.
That's for a regular employee who is having their information confirmed by some hiring manager in a phone or email conversation. This is a press release for a company connected to several very high-profile corporations in a very well-connected business community. Arguably it's the biggest tech exec news of the year. If there's ulterior or additional motive as you suggest, there's a possibility Sam goes and hires the biggest son-of-a-bitch attorney in California to convince a jury that the ulterior or additional motive was _the only_ motive, and that calling Sam a liar in a press release was defamation. As a result, OpenAI/the foundation, would probably be paying him _at least_ several million dollars (probably a lot more) for making him hard to hire on at other companies.
Either he simply lied to the board and that's it, or OpenAI's counsel didn't do their job and put their foot down over the language used in the press release.
Even with very public cases of company leaders who did horrible things (much worse than lying), the companies that fired them said nothing officially. The person just "resigned". There's just no reason open up even the faintest possibility of an expensive lawsuit, even if they believe they can win.
So yeah, someone definitely told the lawyers to go fuck themselves when they decided to go with this inflammatory language.
I wouldn't put money on the last one, though.
You also wouldn't try to avoid a lawsuit if you believed (hypothetically) it was impossible to avoid a lawsuit.
You're assuming they even consulted the lawyers...