The Walmart greeter also isn't paying for the bulk of their healthcare expenses because Walmart provides subsidized health insurance to all employees who work at least 30 hours per week. All US employers with at least 50 employees are required to do so under the ACA. If the greeter worked fewer than 30 hr/wk, they wouldn't get insurance through Walmart, but they would likely qualify for an ACA subsidy that covered close to the entire cost of a health insurance plan on the marketplace.
The statement, "The US spends ~$14,570 per person on healthcare. Japan spends ~$5,790" is about the average amount that the country as a whole is spending per person on healthcare, not what any given individual is paying. Per-capita GDP (i.e. the average economic output per person) is the most relevant comparison.