Pardon my ignorance. But isn't it better to be laid off or fired than to resign, since the former entitles you to a few months of severance pay? Or is it different somehow under these circumstances?
(Or the lawsuit could be a hail-mary on its merits, hoping the employer will settle rather than air dirty laundry in court.)
There may be state-specific laws (in a small number of states for limited circumstances) and there is the WARN Act (but any court payout there would be a penalty on the employer and much delayed for the workers, while it only requires advance notice rather than being like severance -- and also limited by more conditions), but still "no general entitlement to severance" for the vast majority of workers no matter the reason for their separation.