I agree he is thinking EMP.
However, an EMP blast over NK would wipe out most of SK and possibly some of Japan, China, and Russia. That would be an act of war against any or all of those states.
You have not followed what is possible with EMP technology. EMP can do pinpoint strikes over a select area with devastating results. It does not rely on a nuclear discharge to initiate the burst of energy. Chemicals, high capacitance discharges and explosive discharges are what we know without getting into highly classified technology. High Power Microwaves (HPM) can be produced through a special coil device, called a flux compression generator which has been developed since the 1950s. Since the 50's such technology has come a long long way. Classified information about what we have has been closely guarded. Generally, HPM energy can be focused using a specially-shaped antenna, or emitter, to produce a focused field of intense electromagnetic energy which will cut through electronic circuits to destroy semi-conductors which control them. HPM radiation uses shorter wave forms at higher-frequencies This makes it highly effective against electronic equipment and impossible to harden against. This method will take out everything with a single fly over of a cruise missile. Five hundred cruise missiles would render the power of the Pig into the Pig to the pit. Launch sites of possible ICBMs would be rendered useless. Long range artillery weapons would not be able to pinpoint a target. The trucks used to transport ammunition and weapons would be dead in their tracks. And, most important, there would be no communications with which to coordinate a military action.