reading a bit more, I suspect this new device spoofs GPS signals rather than simply jams them.
With simple jamming, you'd know as the GPS would simply stop working. With spoofing, you wouldn't know if you had a valid signal or not. Or where you were (or not...)
I remember around thirty years ago reading a translation of a declassified Russian report (it had been released in a Russian journal) of how they'd spoofed Loran-type beacons around the Black Sea, which ended up with a Hercules losing the plot and getting shot down over Soviet territory. From memory it was a USA aircraft operating out of Turkey on an electronic recce flight- and until the release of the story it was an unexplained loss as the crew incorrectly reported their location right up to losing contact.
If the North Koreans were trying similar tricks it would explain the emergency landing.