The earth is quite connected
Japanese created this awesome demonstration of (hypothetically) a meteorite the size of Honshu hits the Pacific 3000km south of Japan (near Guam). You can see how the fireball engulfs the Himalaya, Amazon, and Mediterranean subsequently.
(Transcript of Japanese narration in English here)
Now imagine the red fire are radioactive dusts, this simulates Curzon's case of nuking North Korea. We may not die on the spot. We may not see the radioactive storm. But the dust travels the same path (see below for realistic comparison).
If we take Kaplan seriously, i.e. more than an average reporter, and do something Curzon's suggested. This may be what we get at the end.
One commentator (robert) said it well, "nice idea. and since north korea is in a perfect sealed vacuum both geographically and politically this would clearly not affect every other country around it." I would add the the connectiveness is more than geography, polity and economy. We have all kinds on indirect connection even with the gaps. This video is one way to visualize them.
Now the realistic comparison:
1) note that it is almost impossible to find an asteroid of this size. The one that allegedly killed all dinosaur was 10 km wide, i.e. about the size of central Tokyo. That was 65M years ago. This one is 50 times larger in linear scale, and 125,000 larger in volume/weight (and hence energy of impact). It is the size of a Jupiter/Saturn satellite (i.e. its "moon").
2) Chernobyl nuke storm spread as far as western Europe, but it was already quite mile when it reached Poland. However, a nuke strike in Korea may lead to much more detrimental results. Japan and Northern China cannot escape the damage. It is not unlike that Pacific coast of North American will not be able to breathe.