One important thing is that the US's response to NK in that book was non-nuclear. They can kill millions of Americans but they are not actually an existential threat. The US did not use nuclear weapons on Afghanistan after 9/11, they used conventional explosives.

Another is that, according to 2020 at least, the North does not have a very robust nuclear command and control communications system, or, being a small country, much in the way of space surveillance assets. They apparently use regular cell phones and commercial imagery.

One of the unfortunate coincidences that kick off the nuclear launch is that the cell phone network is overloaded after South Korea does a missile strike on one of the dictator's residences. From NK's view, missile strikes on official residences and the abrupt collapse of communications, with no reliable information from the outside world, mean that a NATO decapitation strike is underway, so they launch the missiles before they lose them. You wouldn't have the same scenario in the US, simply because we have so many more nuclear weapons and a much longer nuclear command line of succession that a decapitation attack with conventional weapons would take millions of cruise missiles. (North Korea has no backup dictator, while the US has 18 people in the order of succession.)