The NORKS have 10K arty tubes of 152mm and up within 20 miles of the DMZ. One salvo and they’ll kill 50K people in Seoul. None of this requires much movement. The arty sites are stocked and sighted in. By the time USAF and ROK planes are in the air and cany counter battery fire is effective Seoul could be a smoldering ruin.
A lot of these numbers thrown around for DPRK artillery and its effects are greatly exaggerated.
Not all of it is across the border from Seoul, and only some of it can reach the northern extremes of Seoul, and then only a fraction of it can actually reach all of the vast Seoul metropolis.
And cities and human beings are far more survivable than you would think. Seoul has solidly-built buildings and people aren't going to be standing around gawking on the street after the first shells fall.
It will be bad but take with a giant boulder of salt some of the extreme predictions.
There's not much defense against the first salvo, so what are the alternatives?
Which provides an incentive to hit them first.