Well, firstly, the GBU-28 can penetrate to 100 feet of earth or more (granted, it's heaver by a few thousand pounds), so there's no reason a nuke couldn't be built to the same specs. Secondly, the author was talking about a miss with a 10 kiloton device, but the the B61 family of weapons can be configured with a wide variety of yields, including 0.3, 1.5, 5, 10, 45, 60, 60, 80, 170, and 340 kilotons. What you lack in accuracy could be made up for in tonnage.
Secondly, the author was talking about a miss with a 10 kiloton device, but the the B61 family of weapons can be configured with a wide variety of yields, including 0.3, 1.5, 5, 10, 45, 60, 60, 80, 170, and 340 kilotons.
That's correct -- "Dial a Yield" -- and a strange omission.