It seems like a gap in the law/regulation if a former owner (in this case, the pre-Elon Twitter management) can make verbal pronouncements that the new owner is beholden to, without a paper trail. With that said, the complaint does state that promises were made post-acquisition, although this is only mentioned once.
It seems even more puzzling to enforce this on non-profitable private companies. The precedent that would be set here would severely undermine the ability of a private company to operate if it came in far under revenue targets, and was beholden to pay out bonuses to employees.
It's EVEN more puzzling that Twitter, a Delaware corp owned by X, a Nevada corp, is being sued by an employee based in Texas, under California law.