Theory: Xi put massive pressure on North Korea to sue for peace during the armored train trip Kim made to China because he doesn't want Trump/Pentagon using it as an excuse for a more immediate military buildup in the region and the possible nuclearization of SK and Japan in response. Japan was already starting to flex more. Xi probably threatened to cut them off and even invade NK if he didn't cool it, they were becoming too much of a liability. I think China thinks Washington is crazy enough now for a pre-emtive strike (Madman theory?), and that could drag China into a war they are not ready for. The US still struck Syria despite Russian threats, its not unfeasible to think the same applies to NK. China's long term goal is to take back Taiwan, secure their back yard and ultimately become the hub of Eurasian continent, but they need to buy a couple more decades time, they are not yet ready for a true face off. That said however, its also possible this has no effect really, China's posturing itself is enough of a reason for a buildup and Washington can still spin it this way. Like I said, buys more time but probably doesn't negate the inevitable face off. In the end, NK will be authoritarian "modern" like China is "modern". I can't imagine Kim giving up that much power, just shifting it to a better model that keeps him in the loop. This was probably the "out" Xi gave him during his visit. China may even turn this form of authoritarianism into a commodity they can sell to other dictators. "Look at NK then and now! See what our political system / AI facial recognition technology can do for you and your people? Just take a low interest loan from us and we will build the system for you and show you how to use it. You can be advanced just like China AND be in power forever!".
It's truly naive to think there is anything less than geo-political calculus here. If you think this is a momentous occasion, you're fooling yourself with rose colored glasses.
That being said, I'd love to hear some alternative theories. Shoot me down. Pop my bubble.