Depends where in the US they are and what their contracts stated. Particularly pre-pandemic contracts which may have stipulated they should go to the office. In that case it sounds like they could either take the "shut up" money and resign or be terminated for cause.
The company offered a six month severance. Nobody in their right mind would leave that on the table and wait to get fired.
> And why did they resign and not wait to get fired? Is that a us thing?