Their missiles have anthrax warheads and their soldiers are immune to it, probably because they've already been given a vaccine. That means they could launch hundreds of strikes all over south korea while simultaneously launching an invasion with their millions of ground troops and not worry about losing a single soldier to the effects of their own weapons.
What's so hard to understand? It's checkmate. We have nothing to counter this and the our main hope has been to shoot down the few missiles on which he could put a nuke. Now we have to worry about every single theater ballistic missile (of which they have thousands) containing anthrax and we can't even be sure they won't strike the DMZ directly even though they have their own troops there.
Its no more of a check mate than before.