Really they should have just said something to the effect of, "The board has voted to end Sam Altman's tenure as CEO at OpenAI. We wish him the best in his future endeavors."
When you're a public person, the bar for winning a defamation case is very high.
Also, as long as you are a public person, defamation has a very high bar in the USA. It is not enough to for the statement to be false, you have to actually prove that the person you're accusing of defamation knew it was false and intended it to hurt you.
Note that this is different from an accusation of perjury. They did not accuse Sam Altman of performing illegal acts. If they had, things would have been very different. As it stands, they simply said that he hasn't been truthful to them, which it would be very hard to prove is false.
