As long as they are private, they can put lots of restrictions on what can be done with shares. That's the point of the whole article. Ultimately, people are allowed to exercise their ISO's and turn them into shares, they just can't do anything with those shares -- Uber won't buy them, and won't allow you to sell them to anyone else!

I think the sibling post answered the question -- RSU's don't vest until the company goes public. So, instead of getting illiquid comp, you just get none until Uber is public.

ISO != RSU. They are different forms of restricted property, and when it comes to restricted stock, as i said, it's only restricted until vest. They can control when it vests, but even if they are private, they can't control what you do with restricted stock once it is vested, because it is then unrestricted stock, whether uber is private or not.

They actually can if they determine that you possess non-public information which could allow you to gain from the stock in a manner that can be considered "insider trading"

Typically this is only for short periods for higher-up people but it can affect "normal" engineers too (say you do a tech due diligence for an acquisition etc....)