This works if you are in a position to sell the stock to pay the tax. If the company is not publicly traded you need to find a buyer, and because you really need that money before the tax returns you might have to sell at a lower price than the one you were taxed on, if you can find one at all. This makes it harder for startup employees to exercise their stock options before an exit or the company going public, unless they already have cash at hand.

Source: Have exercised stock options in a Norwegian startup. I was lucky enough to have enough cash at hand to pay the tax.

Same as the US. You pay tax on the valuation difference when you exercise. and AFAIK you have to exercise before you sell on any primary or secondary market.

I didn't exercise until we did the IPO, but yeah it might be a problem.

I wouldn't exercise unless I knew I could sell it of course, then I'd be in a pickle or would have had to borrow until I could sell!

The topic though is not really about billionaires, which generally are the founders of the company. It's about the much larger number of multimillionaires that got their money from stock options on IPOed companies. There are far fewer of those in Europe.