The fishing fleets seem to use whatever frequency they want, including offset frequencies within the legal marine HF bands. 6200 kHz to 6525 kHz is the maritime mobile 6 MHz band so 6250 kHz and 6400 kHz both fall within that allocation (technically). Of course, fishing boat radio communications are all over HF. 6400 kHz is another easy to remember frequency, which they seem to like a lot.
The fishing fleet chatter I've heard on 6400 kHz didn't seem to notice the DPRK broadcast signal in AM on the same frequency. Skip zone perhaps? Or perhaps they just don't care. Hiding in the broadcast sidebands is more common then you'd think. Even pirates like Old Time Radio on 6770 kHz often have QRM form pescadores using 6770 LSB or nearby frequencies....