The author apparently does not understand that small nukes are much harder to make than large nukes.
What you do not understand is this scenerio does not depend on large nukes with lots of destructive power.
The story is not talking about a single nuke that could destroy the city of Washington. He is talking about a small nuke that would destroy the White House. It takes two more nukes to get the capital building and the pentagon.
Making big nukes small is hard. Making little nukes little is not nearly so hard.