Earlier quoted context omitted.
Major investors aren't the board's boss in this case because OpenAI is a nonprofit. Microsoft is incentivized to act against the goals nonprofit's charter (and in fact they did in this case).
As much as on paper they aren't the board's boss, based on how events turned out they effectively were. They had leverage over the board to get what they want.
One way this debacle has been portrayed in the media is "an unaccountable board tried to destroy a profitable company". I think a more accurate portrayal is "Sam Altman and Microsoft worked together to deemphasize the company's scientific and humanitarian goals and emphasize building successful and profitable products". It's sort of depressing how Microsoft was able to "capture" a nonprofit.