Seoul is the big juicy target and they have guns that can range it, not many from the DMZ true but how many is hotly debated and it isn't like Seoul is the only place with more than ten houses together. These are shells not nukes, you can drop a few on a small town without wasting firepower the way you would with a nuke.

Probably worth noting that the guns able to range Seoul itself are usually given as around 500 and they are, of course, all large caliber. Meaning they carry more explosives or could just carry chemical weapons. They don't need to flatten Seoul though a couple thousand large shells a minute would do the trick, they could gas them instead.

Doubtless everything they've got that can range Seoul is aimed at Seoul and everything else is aimed at something else, smaller cities and town, military targets, etc. I'm not trying to minimize their rocket capacity or their missiles, conventional or nuclear, but it is foolish to hand wave away the largest collection of artillery on the face of the Earth because not all of it can hit Seoul from the DMZ.