North Korea is on China's borders, thousands of people each month sneak into China from North Korea, and their crazopathic government is next door to China no matter what. I think China being very mildly pro-NK is a case of "Keep your friends close, but your enemies closer." North Korea is probably the biggest potential threat to China if relations were to break down between them.
EDIT also the border between north and south korea is pretty tight
A few thousand unorganised migrant workers who are trying to smuggle themselves in with no more than a suitcase each and try to hide in the corners of soceity when they get there is quite different from a massive uniformed army with tanks and trucks who then try to take over the running of the government when they get here.
Also the army was legally allowed and ordered to shoot the Germans who were trying to come over, whereas they is not the political will to shoot migrant workers.
The documentary mentions that North Korea survives thanks to food from the World Food Programme. Cut all imports, and I wonder how long the régime would last when even the army starts starving to death.
Of course, it's possible millions of people would die in the process.