Also on a re-read, I note that the author was not using housing prices as a measure of affordability:

> Affordability wasn’t hard; I mapped all the census tracts in the country that had a median income within $10,000 of mine, both above and below.

I can't speak to other neighborhoods, but in the case of Capitol Hill I suspect that this measure is a poor proxy for affordability. Capitol Hill residents might be disproportionately people who are willing to pay a premium to live in the City.