It sounds super shady but it's what this company does. It's basically a payoff: "Don't ruin our recruiting pipeline or tarnish the opinion of any existing employees and we'll make your firing a bit less painful with X months salary, just sign here. Also we will let you know what 'X' is next week, after you've signed".

X ended up being 1 week of salary. Though for other people it was 1 or 2 months. Very shitty.