Only because it's unique among developed countries in seeing healthcare costs as disposable income.
You only have to look at the definition in their link to see that US healthcare costs don't get adjusted for.
> most healthcare costs in the US aren't out of pocket anyway.
Right, most people pay health insurance in a way that's indistinguishable from paying taxes in practice (although the US system also comes with significant out-of-pocket costs, so just including insurance costs wouldn't tell the full story). But that "disposable income" metric is defined in a way that considers US-style health insurance voluntary and therefore money spent on that is disposable income (even though it never actually hits someone's bank account in practice), whereas in a country with tax-funded healthcare or mandatory health insurance (unless it qualifies as "social insurance", but normally it doesn't) the costs of that aren't counted in that person's income.
If we look at the income level of the poorest 10%, the United States ranks 16th—not as good, but still among the 10% best countries: https://ourworldindata.org/poverty (click on the Table tab)