The advantage of vesting during a down round would apply only IF employees were paying cash for their taxes instead of withholding shares. Unfortunately, most employees don't have the cash to do so. For equity compensation that is withheld from, employees pay a constant percentage in withholding and have the over withholding converted into cash in the form of a tax refund. If/When they choose to sell their shares later on they must then pay a capital gains tax on the difference between the vest and sell price, effectively double-taxing them on the difference.

The down round is only appealing if you specifically have <1m in shares, have the cash to take a lower withholding rate, and value Stripe exposure so highly that it overcomes both the double-taxing and the opportunity cost of using cash for taxes. That trifecta is a nearly non-existent minority of former employees.