You’ve posted this several times in this thread and it is not correct.

The fiduciary obligation is to act in the company's (not shareholders') best interests, not to maximise (short term) shareholder value. If the board has grounds to believe they can increase the value of the company over the long term by more than Musk's present offer is doing, they're right to do this and are legally safe.

I wish hackernews had more moderation and could ban people for lying

Often repeated, but not true: https://caselaw.findlaw.com/us-supreme-court/13-354.html

Even if that was really a law, which it isn’t…

…what if they board believes they are maximizing shareholder value in the medium or long term? What if they sincerely believe Musk would be a fickle CEO, and would make changes to the platform that would harm ad growth?