The importance of this is often exaggerated. It's significant, but it's not that significant. RAND Corporation modeled this, https://www.rand.org/pubs/research_reports/RRA619-1.html

It assumes ~130,000 casualties from a worst-case surprise attack on population centers by the North.

If a conflict started ramping up, evacuations would rapidly shrink this.

A significant deterrent, sure. But it rapidly becomes less and less meaningful as the DPRK builds its nuclear arsenal.