The point is that billionaires are able to restructure their activities so that more of their income is in the form of capital gains, which are taxed at a lower rate than salary income.
And capital gains are not taxed at a flat rate. The federal capital gains tax has three rates. The top rate of 20% applies only to taxpayers whose incomes, including capital gains, exceed $500,000. It's actually a bit above $500,000, but I don't want to look it up. Only after that income level does the rate become flat.