I'd argue that more than 50% should be affordable to a median buyer, on the basis that not everyone lives in the most expensive house they can possibly afford,

I'd argue that more than 50% should be affordable to a median buyer, on the basis that not everyone lives in the most expensive house they can possibly afford, and in the middle of the house pricing range there are multiple homes available at any given price. Of course, there's also a large set of people who cannot afford to be buyers [in areas suitable for them to live] for various reasons and this median income dataset includes them. Not entirely sure they've used median household income and not median individual income either