As I understand it, you have to pay tax on the difference between the option price and the value at the time you buy them. So if you have a bunch of options to buy at $10 per share, and the company grows to $90 per share by the time you quit/have to buy your shares, you're taxed on $80 a share. Remember, stock in a private company is worthless until you find a buyer, which is one of the reasons the people referenced in the article had to go into debt to exercise their options.
30% of a large amount of imaginary money ends up causing a gigantic tax bill, paid in actual money.