I came to that conclusion as well. China will let them go after Japan.
I don’t think they actually will let anything happen. I just believe that the DPRK believes it. (previous comment was attempting to see the view of the DPRK). I believe China needs stability and the DPRK is testing the waters.
Think about their series of threats.
First they officially declared the cease fire over with the South. Nothing happened.
Then they openly threatened us (posting pictures of US targets). Nothing happened.
Now they are threatening Japan.
I think they are seeing who they can provoke to take preemptive action for some reason. Maybe they think China won’t turn on them if they are attacked first. Of they are just seeing who may pay them off.