How about just basic energy inputs like fuel and fertilizer? This is a country that is so abysmally poor the state has a feces collection quota. Peasants must collect their daily dumps and give them to the state for use in the field or suffer the wrath of the local DPRK gangster.[0]
Perhaps that could have been true for the shipments of shells, but I can't see NK sending tens of thousands of troops to a meatgrinder that they have absolutely no stake in just for some fertilizer and oil. NK has the ability to demand much more strategic things and I'd be virtually certain that they have.
[0]https://www.rfa.org/english/news/korea/manure-02012022130916...