Supply lines require supplies
North Korea has minimal supplies to line
North Korea simply cannot sustain offensive military operations and their whole military and governing philosophy has been to extort South Korea with artillery threats to Soul from artillery emplacements along the DMZ, which requires no mobility on the part of North Korea
That such an attack on Soul would mean the end of North Korea has been a given for the 30 years since the fall of the Soviet Union
North Korea is trying to follow it's extortion model to make the entire world Soul by threatening nuclear ballistic missile attacks on the worlds capitols and population centers with massive assured destruction if it does not get its way , just like North Korea has gotten its way for 60 years by threatening massive assured destruction of Soul if the world does not accede to its demands and extortion