The board is being given a sanity-check; I would expect the signers intentionally left themselves a bit of room for escalation/negotiation.
How often do you win arguments by leading off with an immutable ultimatum?
200 people or even 50 of the right people who are definitely going to resign will be much stronger than 500+ who "may" resign.
Disclaimer that this is a ludicrously difficult situation for all these folks, and my critique here is made from far outside the arena. I am in no way claiming that I would be executing this better in actual reality and I'm extremely fortunate not to be in their shoes.