BWI is, as the name implies, right between Baltimore and DC (which are about 40-50 miles apart). No matter which city you're bound for it's a hike, but usually cheaper than Dulles or National.
Fun fact: on the morning 9/11 happened I was packing for a fllght to BWI. They made me give it back though, the employer claimed they needed it to get a refund.
Fascinating news about this flight though. Back in the '70s I took my first trip to Europe on Icelandair, which was offering a similar deal. Don't remember what the price was but it was damn cheap, you stopped at Keflavik (Rekjavik) and continued on to land in Luxembourg -- which is kind of a BWI of northwest Europe. They left the US out of Newark.
I always regretted that I didn't get off the plane in Rekjavik and spend some time before continuing on. This is definitely definitely food for thought.
