If that were true would it make sense to let them know ?
It weren't true then you might be able to cause a good deal of mayhem by suggesting that it was. Brother Kim appears to believe that he's surrounded by traitors and this might be quite a good way of getting him to use some of his better IT guys as anti-aircraft gun fodder.
If it were true then you'd probably keep quiet about it until the fireworks were set off in anger. Then you'd arrange for them all to crash in the suburbs of Pyongyang and you'd solve all your problems at a stroke (shame about the civilians though).
Of course the idea that US hackers could usefully penetrate a defence IT system in one of the most inaccessible countries on the planet is just laughable. Oh ... hang on ... https://en.wikipedia.org/wiki/Stuxnet.