Sounds to me like it's going to be either take the "voluntary exit" with the severance, or if you don't then eventually get let go without one.
The part that can't easily get jobs elsewhere?
If I was in that bucket I would definitely take this offer.
Looks like all the large tech companies are doing aggressive stack ranking right now.
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)
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.
Depends on their employment contract, I don't think it's necessarily guaranteed.
I'd assume you have to agree not to sue them in order to get it though.