I don't believe this is true.
I was listening yesterday (Nov 9, 2017) to 6400 kHz at 1542 UTC on the Univ of Twente's WebSDR. I did hear Europe's
The Final Countdown, followed by
Take It to the Limit, followed by something else that I do not recall. The second song started to fade and another station was coming up, and that sounded like the North Korean station with their Cold War red music (which was consistent with the music on Voice of Korea).
Several times, I distinctly heard two stations on the air at the same time.
FWIW, I was chatting with someone at work and I mentioned this (time in CST, since that's where I am):
09:42 Pyongyang Broadcasting Service is on 6400 kHz with music or something. There's a second station playing "The Final Countdown" by Europe. lol
I recorded it, but decided not to save it as it was uninteresting. Guess that was incorrect.
If the DPRK had two transmitters on the frequency, I suppose one could've been hijacked...