Which is a breach of their fiduciary duties.
The issue is indeed always to determine whether a course of action is knowingly not in the interests of the shareholders.
Fiduciary duty is very much a thing. The Twitter board with such a tiny amount of equity may incur risk from "honest services fraud," if they block shareholders from recieving the material and obvious value of the offer.
They can’t argue that because it would be completely irrelevant to current shareholders.
They can argue that the long term value is higher than Musk's offer.