The job market is terrible right now. More likely the high performers will take the offer because they believe they have better chances of being rehired somewhere else. Good luck to them though.
Assuming the events are independent, P(you get laid off) x P(you don't find a new job) will always be less than 1.0 x P(you don't find a new job)
If I already had a solid offer somewhere else, I'd take the severance. If I didn't, I wouldn't leave it up to chance, and I can't imagine many other people would.
With only three weeks to make decision, my guess is high performers aren't going to start looking for new jobs now unless they were already looking to begin with. From the wording of the message, it sounds like Google is okay with losing the group that was already looking.
Assuming the events are independent, P(you get laid off) x P(you don't find a new job) will always be less than 1.0 x P(you don't find a new job)