I believe they are counting all taxes, not just income taxes. Sales and excise taxes often hit the poor very hard. On average, across all states the bottom 20% pay 10.9% of their income in taxes while the top 1% pay 5.4%. In Washington, which has no income tax and a high sales tax, the poorest 20% pay 16.8% of their income and the top 1% pay only 2.4%. [1]

[1] https://itep.org/whopays/#The%2010%20Most%20Regressive%20Sta...