China needs food, lots of food. China is 12.5% arable land, but much of the best and most readily accessible is overfarmed or severely contaminated. NK is 19.9% arable land, most of it untainted by industrial activity.
I have a suspicion that Xi gave Kim a long, meaningful gaze, and respectfully asked Kim to stop barking at his customers and make him a sandwich, or he'd gladly find himself another snarling, mangy mongrel to chain in his front yard. One that knew his place, and his master.
Overdue, if so.