Well first if you are going to use Bill Gates as an example you should know a little bit more about Bill Gates. Bill Gates is a very liberal guy. He's on record as saying he doesn't plan to leave any of his money to his children and plans to devote it all to charity just as one example. His father, who's also the head of his foundation, is on record as saying he is in favor of and 99% tax rate. So Bill Gates really doesn't care.

But on the topic of just your ordinary super rich person I think you're right. One or two percentage points won't make a difference to them. It certainly won't motivate them to leave the country. But what this author seems to be suggesting is that taxing the super rich can solve all of society's ills. In order to do that you need to tax them far more than just a couple percentage points. And everyone will have their threshold. The super rich can afford to fly back and forth all they want so eventually they're just going to move their official residence elsewhere and fly back to the United States when they need to.

(To give just one example Arnold Schwarzenegger literally commuted by plane during his second term as governor of California. He would fly, via private plane, to Sacramento in the morning and then fly back to Los Angeles in the evening)