There's the WARN Act, but I don't think that impacts what happens after a duly-warned layoff, unless I'm mistaken?
Technically the company can't get around the WARN act through severance but it is essentially pre-paying the damages so there's no reason to go to court.
I wonder how many his employees in Tesla/SpaceX/where ever are currently incentivised by stock options, who’d watch their boss doing that to Twitter people and wonder why they bother working evenings and weekends, if the options can be yanked as cruelly as that?
Also, because of being outmaneuvered, he may be seeking any chance to stick his finger in the eye of the people who did it. That the logical consequences will be that his finger gets cut off... well, as I said, I'm not sold on his foresight at the moment.
Elon is interacting and trying to dodge this act. He seems to want to cut Twitter's workforce at a scale that the WARN act covers.