The party congress was stacked with new Xi promoted middle level PLA officers during by during Xi Jinping’s speech.
My guess would be that the Xi Jinping doesn’t want anything bad to happen on the Korean Peninsula that would screw up his plans to take Taiwan in a couple years.
I have read that they believe the war in Korea in the early 50’s stymied their efforts to take Taiwan subsequently.
So instead they skirmished with the Russians for 15 years.