Note that most people currently paying a mortgage has not bought at current prices.

And with the reasonable assumption that high-income households are more likely than low-income households to buy a house in any given year, we can expect the buyer of the median house to have better-than-median income.

It makes more sense when you put it like that. The average mortgage holder has a 2019 salary but probably only a 2010 mortgage. (Or whatever bygone year is correct.)