I don't believe that NK's nukes deter the US from doing anything. Would NK nuke Guam and risk getting carpet-bombed with nukes for endless days and nights until even the ants are dead? Artillery on Seoul doesn't matter. The US would just ask SK to evacuate it.
The US doesn't do anything about the DPRK because it's not economically relevant (i.e. it doesn't have the world's largest oil reserves etc). In an ironic way, their economy being closed-off and mostly unintegrated with the Western world maintains the peace.
They are the North Korean leadership saying that if the US (or China or anyone really) tries to surgically decapitate them (like the US just did in Venezuela) then the nukes are used to take the attackers with them
How do you evacuate 10 to 15 million(counting Incheon in) of people, fast? Where to?
It assumes ~130,000 casualties from a worst-case surprise attack on population centers by the North.
If a conflict started ramping up, evacuations would rapidly shrink this.
A significant deterrent, sure. But it rapidly becomes less and less meaningful as the DPRK builds its nuclear arsenal.
