I used to model nukes for the DoD. Doctorate in molecular physics.
Then you must know that it would take either a very large thermonuclear warhead, or several dozen (or tens of dozens) 100-400kt warheads burst at altitude to get the effect? Even Russia told us they 25MT warheads on their Mod3 SS18s were for taking out LCCs and they wouldn't waste one of them on an EMP.
NORKS may have the pistol, but there is only one round in the chamber.....and its wet.
EMP sending us back to caveman days is beyond hyperbole.