Just as the board and executives has fiduciary obligations to shareholders, employee option holders fall into that population as well. There are legal incentives/threats against not handling it fairly.
The rule of law is a good thing.
It's up to what Musk wants to do with the existing workforce, but 'promising' existing plans/compensations will be upheld for a period gives him time to introduce some kind of option plan in the new Twitter holding company to retain staff he wants/needs and sack the rest before there's an exodus not of his choosing.
If they're even there in 6 months. Musk can just fire them all and do whatever.
YMMV, but that's what lawyers are for. Not the action of a single individual necessarily, but a group of disgruntled employees can pool their resources and hire a law firm to right a wrong. It's usually worth it when the "wrong" is big enough.
There has to actually be a wrong, though. Nobody at Twitter is entitled to their job for any period of time beyond today - anyone except probably the CEO and a few other C-suite folks can be let go with about ten seconds notice. And I'm 100% sure there are clauses in everyone's RSU contracts detailing what happens in the event of an acquisition.