If they had written consents from a majority of the board to remove Altman and Brockman from the board, then depending on the applicable nonprofit law and corporate governance documents, the board removals may very well have been legally conducted without need for a properly noticed board meeting. (For the actual firing of Altman, that might have been legal either through written consents or through a board meeting after the removals of Altman and Brockman.)
Having no information on what laws and governance documents apply to OpenAI or on what steps the board took, I express no opinion on whether the legal requirements were actually met, but it’s possible they were.