North Korea is angry at China for not coming to their aid over recent war crimes accusations. North Korea is even angrier, and very shaken that a retired Chinese general said [publicly] that China would not come to the aid of the current North Korean government if the government collapses or starts a war. China often makes official announcements via public “comments” by retired senior government or military officials. This makes it easier to, if need be, back off from the new policy. China has not backed off this one. China is telling North Korea to do what China wants or else. China wants work on North Korean nuclear weapons stopped.
Distancing themselves from North Korea is nice, and all. But right before the Korean War we placed South Korea outside of our defense perimeter yet still intervened quickly when the North Koreans--backed by the USSR and China--invaded South Korea.
So if North Korea goes belly up and South Korean troops possibly aided by American forces cross the DMZ to protect Seoul from artillery bombardment and to grab North Korean nuclear facilities, don't be too shocked if China intervenes anyway to prevent a perceived hostile force from marching up to the Yalu River.
Remember, the Chinese just won't intervene to protect the current North Korean government. That's a bit different than saying they won't intervene on the peninsula.
And the Chinese could change their mind, of course.
As an aside, I wonder if China let semi-official hackers inside China go after Sony for North Korea as kind of a consolation prize of weakened Chinese support for the North Korean regime.