Then he shouldn't have signed the severance deal (which he didn't). An employer is not entitled to give you money to go away; they give you money in exchange for certain things, one of those being that you shut your mouth and not do an AMA on reddit about it.

This guy is an idiot and is likely going to have a hard time finding a job based on this behavior. What he's doing isn't illegal, but it's quite unprofessional.