He used median income. Rich people in the DC suburbs tend to live in mansions, because they live in the suburbs. [Nouveau-]Rich people in the Mission live in 1-bedroom apartments because they're hipsters.
The DC suburbs include some of the richest neighborhoods in the country, yes, richer than San Francisco (where do you think $2T annually in government spending goes?). Mill Valley also probably has a higher median income than the Mission, despite cheaper 1-br apartments.
DC suburbs themselves aren't more expensive than SF or the Valley though in general, speaking as a recent DC transplant to the Valley. You can find a 1000 sqft 1 bedroom apartment just outside of DC and a few inside DC for about $2000 a month. Go a little further to even Fairfax and you can get a 2 bedroom one for even $1600 if you look carefully.
While I haven't lived in the Valley long enough, I'm currently paying about $2100 a month for just 700 sqft, and other places are even worse around here.
And income may be higher in some respects, but that is pretty much exclusively in the government sector or law.
The DC suburbs aren't rich so much as they're segregated and uniformly upper middle class. E.g. Fairfax County is one of the wealthiest counties in the country, but almost nobody here is "rich." Its a bunch of older dual income couples making $200k combined, with very few people lower in the income spectrum.
The DC suburbs include some of the richest neighborhoods in the country, yes, richer than San Francisco (where do you think $2T annually in government spending goes?). Mill Valley also probably has a higher median income than the Mission, despite cheaper 1-br apartments.