This is based on
ordinary spending, not
essential spending. It has nothing to do with frugality. Transportation expenses are "ordinary" without regard for how much someone chooses to spend on a car. A lot of luxury spending is captured in "ordinary" at the aggregate level.
The US Bureau of Labor and Statistics (BLS) tracks the expenditures of every income decile of the population in considerable detail. If you want to know how much the average person at the 30th percentile of income spends on eggs, fresh vegetables, commuting, or many types of entertainment or non-essential spending, BLS can tell you in surprising detail. I have not seen similarly detailed data for other countries, which makes comparisons difficult.
The expenses BLS defines as "ordinary" are comprehensive, pretty much everything you'd expect, and they make no value judgments regarding how people actually spend in a particular category. The only category exclusion from ordinary living expenses that might be controversial is eating out at restaurants, but that is based on the fact that most Americans do so infrequently (it won't feel this way to the urban minority).
Subtract the median ordinary expenses from median income to arrive at $12,000 per year. Anecdotally, this number feels about right.