Well, I mean, it getting hit so bad would be why its that messed up to begin with, and why the water is as toxic as it is even 200 years later. Obviously the nation's capitol would be one of the hardest hit areas in the country, if not for taking out political leaders, then for the demoralizing effect of the destruction of major national icons.
But I dont think they need the "no life for decades" thing for it to work.
IIRC, the guy in the Dunwich building has a note talking about a time so soon after the war he didn't know what ghouls are, so that always seemed to indicate to me life was there even shortly after the bombs.