From reading the article, I don't think that this is an accurate description of the situation.
I believe that Twitter employees have been getting a discretionary bonus, so normally Musk would have been fine simply not paying it out, but as part of the acquisition he promised that he/the company would pay it out as it had been in previous years, with some stipulations, etc.
So the issue now is that he promised he would pay it, which means that he's obligated to do it, because the employees made decisions based on that promise that cost/lost them money.
It seems like very simple promissory estoppel.