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.