Likely that contract doesn't have a clause that states it only remains valid if the employee clicks on "yes" in death march emails.
So, the contract remains valid until either the employee quits or the employer terminates. Not reacting to an email is not quitting.
I'm pretty sure that legally, the employees that received that email and didn't click/answer are still considered employees. They are only terminated once they received their actual termination papers, not an "if / then" email.