And as far as I know NK never signed any agreement to be legally under the jurisdiction of the international body that forbids nuclear underground testing.
All the rhetoric is not going to cause any war. All KJU wants to do is to test and build nuclear missiles for defense -with or without rhetoric ( and get publicity) -He doesn't want to wind up like Saddam Hussein. But, at least for the immediate present KJU is not going to attack the US -with or without rhetoric. Rhetoric is not a factor for KJU.