They could still feed themselves if they rewarded agricultural work - and fishing - sufficiently. That would mean higher pay for farmers than for urban government workers or soldiers. Instead, they have the typical extractive economy of the communist model, where a few cities try to live off the surplus produce of the countryside, as old nobilities lived off peasants. But nobilities were very small. Modern bureaucracies aren't, nor are mass armies (NK military mobilization is the highest in the world).
They can't pay the farmers very much while supporting all their party members and regime supporting soldiers. So they substitute force for consent, trying to get the extracted pay of the rest without commensurate reward to farmers. This works only in the shortest term. Because it makes everyone prefer a city life to a country one. The fields are deserted. Output plummets, because it is all going to those who do nothing to produce it, and producing it brings no reward. At first they can have soldiers order the peasants around more harshly, restrict their movements, make them live under goons with guns all day.
But that does not increase output. The goons eat and do not farm, and the farming proceeds in the worst possible conditions. Men try to run away, to China e.g. Or to the cities. They bribe everyone they can. If they have any family or connections in the cities, they use them to get out of the whole penal farming system. The regime responds first by sweeping up people in waves of arrests and sending them to labor camps - formal slave labor in place of regulated serfdom. The distinction between a prison camp and the countryside at large blurs, disappears.
Output keeps falling, the extractions do not stop, so peasants and prisoners starve to death. Those in the cities, or the guards, eat what food their is. Soon the prisoners are dead or escaped, the guards are doing the farming, badly, because there is no one else. But they can't. So they go to the cities and grab the party members, the lower ranking ones, who fed off the whole thing, and order them out to the fields to take the place of the peasants they have killed off or driven away. They can't pay them, so they order them under guard. Guess how productive city clerks will be as farmers.
So they beg for aid by proxy, to keep the whole thing afloat. Absent outside assistance, the whole system will crash into chaos in a few years at most.
Nice summary.