You think they are doing this from NK ip blocks/asn? Their physical links are more or less enemy of enemy with US, so they have no incentive to block. Its impossible to keep them off the internet.
All the “international sovereignty” responses are humorous to anyone whose paid attention to the last twenty years of the American military.
Is it technically feasible? Yes.
How do you imagine that a drone will be able to find and damage an underground cable?
This is not Afghanistan, a drone will not live long past China’s/Russia’s air defences.
I have no doubt the US has plans to cut them (and probably anyone else you can imagine) off in the event of war.
So it might not be a great idea to start doing stuff like that in their back yard? ;)