Earlier quoted context omitted.
That’s what Wiener suggests in Uncanny Valley, when she describes the windfall when Microsoft bought the GitHub shares she had the option to buy during her time there. It wasn’t life-defining, plus she noticed that many other employees didn’t have enough savings to actually exercise.
The way it should happen with a liquidity event is that you simultaneously exercise your options and sell them, pay off the taxes and bank what's left over. Savings should only come into it if you're leaving before the liquidity event.
Which is another way some startups screw people. If the company is not willing to budge on the "standard" 90-day exercise window, there shouldn't be any negotiation. Just walk away.