The point is: there are a ton of people in that area. In the abstract, putting two offices within 250 miles of each other seems silly, but given that it's the densest part of the United States, there's enough population to justify two major offices. Drawing a circle around DC is just a vivid way of illustrating that. It's not totally abstract either: while many people in tech move across the country to the Bay Area, moving a few hours for a job is much more common (you can still drive home to visit relatives, etc).
As far as the actual numbers, thanks: your source is better than mine. I think you need to extend it to 250 miles to get Queens and DC in the same circle, at which point it's 18%. I still believe that there's no other area with nearly that density in the US (circles centered on Dallas, Chicago, San Francisco, LA or anywhere in Florida are at least 20 million fewer people).
The land area doesn't really matter that much.