I am fairly confident that you have strong legal protections against forfeiture of your RSUs under these circumstances, especially if you worked in California. Unless there is an explicit clause in your employment contract where you agree to this (fairly unusual) tax prepayment scheme, your RSUs are earned income.

IANAL, but I have been around the startup scene long enough to immediately spot the red flags here. It seems like there is an explicit strategy at play here to discourage former employees from exercising RSUs, and you will likely find a strong legal argument against these terms (i.e. prepayment and forfeiture) unless you explicitly agreed to them. To echo what will probably be stated at least a hundred times on this thread - talk to an attorney.