Proof that it's a myth? Because that is the cornerstone of North’s deterrent and ROK government holds monthly (IIRC) civilian shelling drills at Soul.
This is a good beginner's reference.
http://www.militaryphotos.net/forum...40-Bluffer-s-Guide-North-Korea-strikes!-(2009)
All you have to do is look at a map and look up the range of artillery. Seoul is well beyond the range of all but a fraction of North Korea's artillery. Civil defense drills are prudent, but doesn't prove the myth that the North can level Seoul. The North can do some damage for a few days before their arty gets suppressed. But this is survivable whilst their nukes provide the real threat.