There's a $19B difference between Twitter's market cap and the agreed deal proceeds.
Twitter has to pursue receiving that-- either in consummation of the deal or adequate recompense. If their board doesn't, they'll be sued into oblivion by their own shareholders. It's do-or-die.
That doesn't mean, though, if they were offered a couple billion less for Twitter and were sure that Musk would then close... that they might not blink, rather than wobble $19B+ on a court case.
The problem is, a $2B discount on Twitter doesn't improve Musk's financial condition much or make the deal materially less stupid for him.