It's not? Really? Think about that very carefully, and consider what the explicit threats that NK has made to both Japan and South Korea implicitly mean for China. When a bully waves a gun at a couple of people, don't think that he's not waving it in everyone's face.

Do you think China will be sitting pretty if worst comes to worst and a few dozen nuclear warheads rain down on Japan, South Korean, North Korea by intent, and on other countries by accident?

Sure, China can try to command the wind not to blow clouds of radioactive fallout its way, but will the wind obey?

https://en.wikipedia.org/wiki/King_Canute_and_the_waves

Is China willing to gamble on the weather?

20% of China's agriculture land is already suffering from poisonous industrial metals pollution. Dare China see another 10% contaminated by radioactive fallout as well?