I think his talk is more habit.
The last few dictators talked that way and no one took much notice of them. More of the same from this one. If he were to actually try something, you're right, most of the world would come down on N.K. like sh!t from a sea gull.
Sometimes I think they talk that way because of their location. S. Korea on one side and China on the other. Nuke one and you're liable to piss someone off. Tactical nukes or larger thermobarics on Pyongyang and selected other sites where NK nukes are concealed might have to be deployed.
I'm sure the major powers have discussed this to death (no pun intended :lol: )