Billionaires get richer through asset appreciation, not income. The White House report is focusing on how fast the rich are getting richer and that's how they arrived at 8%.
A worker's salary is taxed at a high rate like up to 40%, whereas a wealthy person's gains from assets are taxed at a much lower rate like 15%. And that is if they realize the gains at all, because their unrealized gains don't get taxed at all.
Our current system (in which long term capital gains tax is much lower than income tax) rewards the owner class and punishes the workers.
The figures are misleading because if you have low income, your effective tax rate is indeed lower than the wealthy. But then you're barely surviving anyway. So if you're poor, you're screwed because you're poor. If you have a livable income, you're screwed because of high taxes. But if you're wealthy, the system exists to serve you.