Artillery typically does not shoot that far.
I read the article back in April...
From the article:
Pyongyang has an estimated 4,000 artillery guns and rockets placed on the heights north of Seoul just across the DMZ, many of them on rails so they can be moved into place in time to avoid detection.
According to the article, the North Koreans woulc cross the DMZ after the initial attack. They have quite a few rockets and missiles -- some that reach Japan...
And it is possible North Korea might put Chemical Weapons on the rockets/missiles...