Tell Kim Jong Un. Being in control of a basket case country has not stopped him sabre rattling. Mutually assured destruction only has to not work once.
The DPRK-USA relationship is most definitely
not an example of mutually assured destruction. There is nothing mutual about it.
They have, between a dozen and twenty tens of kiloton class weapons. The USA has several
thousand megaton class weapons. Kim Jong Un is going to sabre rattle, but the only way he will actually use his nukes is in a scorched-earth role. Despite the rhetoric from the US administration, absent an attack (or belief that an attack is under way) the North Korean nuclear arsenal is not a threat to South Korea, the US or any of their regional partners.
An India-Pakistan nuclear war is a greater threat.