"This action for equitable relief arises out of an effort by Wachtell to fundamentally alter its fee arrangement as litigation counsel in the twilight of its representation of Twitter to obtain an improper bonus payment in violation of its fiduciary and ethical obligations to its client," claimed the lawsuit filed by X Corp., the successor company to Twitter. "Wachtell exploited a corporate client left unprotected by lame duck fiduciaries who had lost their motivation to act in Twitter's best interest pending its imminent sale to Elon Musk and his entities, X Holdings I, Inc. and X Holdings II, Inc."
It sounds like the "lame duck fiduciaries" were quite effective. Those fiduciaries hired this law firm to secure the acquisition that Musk promised for the benefit of the original Twitter shareholders. Had these fiduciaries not done so, the deal would possibly not have happened and the original Twitter shareholders would have lost substantial value. The fact that the fiduciaries paid out when the deal was imminent to close shows even more that they asked for a legal service, received said service, and paid their bill.
Musk is just upset he didn't get to deny the payment to the law firm that force him to honor his obligation to buy Twitter at the high price.