The rumor is something like 75%, which would definitely explain the panic actions. This also means they don't have to specifically cut off badges for laid off people, it gives them time to decide who's actually gone.
Checking that box was agreeing to work “hardcore” and add a bunch of hours to your day. I imagine a lot of people receiving that email guessed that their colleagues would also ignore the box and figured “he won’t fire all of us, right?”.
Hey Elon, how about this: fire employees with long names, because they take longer to type in their login, and that's inefficient!