The equity explanation makes a lot more sense. If sama was "not candid" to the board about being the ultimate owner of OpenAI and his goal is to get unbelievably rich (he's already a billionaire and obsessed with money) they would have to fire him on the spot. Sam would immediately try to move against the board so they can't give him any window to make a counter-move. And if that means firing him on a Friday right before markets close, so be it.

If Sam made a deal with MSFT that required board approval they would be mad, but not this mad. The board feels betrayed, and Sam being the secret owner of OpenAI through the foundation checks all the boxes.