Agree on that, this Ships will not be much a match to any ROKN new Frigates let alone Destroyers. However DPRK Navy still need to provide updated surface vessels. Considering their situation, those ships will be good exercise for their Shipyards. Perhaps in similar ways that Myanmar domestic build Frigates. Both isolated regimes and need to shown their Military Production to continue support their hold on to power.
Besides with the amounts of VLS they carry, I don't think DPRK plan for them to face ROKN vessels directly. More likely they are going to operate on coastal waters and launch salvo of their missiles, being covered by DPRK aerial assets. It won't make them Green Water Navy and still stay as Brown Water ones.