Once they leave the company they are free to sell the stock. They are not beholden to a stock blackout anymore.
The employees can't sell these until the blackout is lifted. Currently there is no end date set.
Edit: Per below comment, don't do this :)
If you could find someone to lend you cash (based on the totality of your financial situation and personal credit), that's fine. It's not fine to borrow someone else's shares and short them during a blackout.
If you have MNPI you can't buy or sell anything, including stocks or options, even if you don't work for the company.
Do they contain penalty provisions beyond getting fired? They're meant to protect both the company and the people from the slightest risk of insider trading allegations, so in most cases violating the policies isn't actually illegal insider trading.