If Kim Jong Un initiates war or conflict with anyone his dynasty will not survive no matter what the outcome and anyway, It would be absolutely impossible for Nth Korea's pop of 27m to forcibly subjugate the south's 57m, not even in George Orwell's dreams. I certainly have no idea what Kim's short term goals are but I am pretty sure in the long run he doesn't want to go down in history as the very last of his dynasty, and he well knows if he mucks up what's been handed to him on a silver platter with all the trappings he'll end up dead and forever humiliated, and probably his entire family also.

Same is true for Xi. There is no way China could ever successfully complete an invasion of Formosa across the Taiwan Straight, and if Xi foolishly attempts it will mean the end of him personally and Mao's party.