If Stripe had offered 83(b) and issued cash bonuses to cover exercise costs, would this tax issue been avoided?
It sounds like Stripe is going to declare a qualifying event which will convert outstanding RSUs into shares which will bring along with it a major tax implication for the RSU holders, who would be in a really tough spot if they got a windfall of totally illiquid Stripe equity. So Stripe is going to buy shares to facilitate tax withholding, for which they need the $$ they are raising.
I may have misread, but I think OP implied everyone would have been better off with options. There were a few years when RSUs were the craze, but they mostly screw holders.
Other reports have said that Stripe is raising to cover RSU tax bill that would come from letting them vest in order to prevent the RSU from expiring.
That said ISOs have similar risk but additionally you could lose whatever you paid to exercise and also you could incur AMT which complicates your tax return quite a bit.
Now, like all income if you set your withholding incorrectly you can owe at the end of the year.