Hard to say. It looks like this article has been republished in several different outlets but originally came from the AP[1]. At first glance they appear to cite the statistic as coming from the EU Tax Observatory, but it's worded in such a way that the provenance may only apply to a prior fact:

> In the United States, [the EU Tax Observatory] says, billionaires pay an effective average tax rate of 23%, including all taxes at all levels of government. The poorest 10% of Americans pay more – 25.6%.

I was unable to locate any such figure in the Tax Observatory's data atlas[2] -- I've included the relevant lines for their latest U.S. effective tax rate estimates below. As you can see, they're not broken down by income percentile and also lack the expected decimal precision:

    USA 2020 0.31 Total_tax_NDP
    USA 2020 0.11 Corporate_tax_rate
    USA 2020 0.27 Labor_tax_rate
    USA 2020 0.30 Capital_tax_rate

It's possible that the statistic is instead buried in one of their publications[3], but I was not able to find relevant mentions of "25.6"/".256" in any of the recent reports. This particular NGO seems mostly focused on EU corporate taxation rather than American labor taxation... so I wouldn't be surprised if the fact source is actually some other uncited party.

[1]: https://apnews.com/article/tax-corporations-global-havens-eu...

[2]: "Effective tax rates on capital and labor": https://atlas-offshore.world/download-data/

[3]: https://www.taxobservatory.eu/publications/