So, another comment detailed other legal aspects, I'll tackle the "most" part. Even if no other contract exists, that means at least 50.1% of all laid off employees should receive some type of severance. Said severance can be anywhere from a penny to millions.

We don't know what went down. Musk could have actually paid 50.1% (or greater) employees severance. It doesn't matter, because in the U.S. you can sue anyone else for anything, illegal or not. I can sue you because your username sucks, for example. Oh and if you don't show up, I win a default judgement, so there is that.