Wouldn't they be taxed at the bonus rate when they vest (22%)?
The IRS gives you some time to pay the difference; I think if they get their money in the tax year, they're happy. So if you owed those taxes but knew you'd sell in November, the loss would offset the gain, and it'd be like you never owed the taxes. It would work out so you'd pay taxes on the sale price as though it were all short term capital gains. The problem comes if you don't sell in 2019. First, you'd owe those taxes on gains you never really saw. When you realize the loss, you'd be owed the taxes back, but as an offset to gains elsewhere the year you sell, or in $3000 increments for the rest of your life.
37% is the highest federal marginal tax rate. It starts at $510,301, which might kick in for a decent number of Uber shareholders at IPO.
That's also the tax rate, not the withholding rate.
I have no idea what they meant by "but likely need to pay a lot more than that yourself," unless that was a reference to state income tax. 37% is the highest bracket. It's higher than AMT. I'm not sure under what circumstances you'd owe more than 37%.
You were taxed the minimum amount (37%) when they vested, but likely need to pay a lot more than that yourself.