That's not how ISOs work. You are taxed on the bargain element (discount to fair value provided by the option) at exercise under AMT, and taxed when you sell under the normal tax rules.
Let's say you worked at Uber and you exercised ISOs at the IPO that had appreciated such that the bargain element at exercise was $1,000,000. You would owe 28% Federal + 7% California = $350,000 under AMT (ignoring a lot of other things for simplicity) in April 2020. When you sell your shares, you will be able to reclaim some of this tax through credits for AMT paid in previous years.
By the time the employee lockup expired in November 2019, the shares are trading 43% lower than the price you exercised at and are only worth ~$577K. You still have a $350K tax bill under AMT due in April 2020.
Because you exercised in 2019, you have an out - by selling the shares in a the same calendar year, you can lower the basis under AMT, but you give up the ability to pay long term capital gains rates on the sale (0-25% fed + 0-13.3% CA) and instead pay ordinary income tax rates (22-43% fed + 0-13.3% CA). Let's just say it would be 40% - you could reduce your tax bill to $231K by selling all of your shares.
You have until the employee trading window closes in mid-December to decide whether to hold onto your shares for the eventual preferential tax treatment, hoping they recover before the tax bill ruins you, or to double your tax rate to save money on taxes this year.
Now, the benefit of hindsight:
If you had sold all of your shares, you would have missed out on $330K in gains in the following 60 days. If you held all of your shares, you are still facing a hefty AMT bill and years of careful tax planning to reclaim the AMT credits you generated. Oh, and even if the shares 10x in value, you are still going to have >$100K in AMT credits that will probably take 10+ years to recover.