True, but there are other considerations.
If you build more boats you have to man them. More men, more training, more space at the docks for berthing them.
And how many boats do you need? ROK on needs enough boats to keep an eye on NRK’s boats.
If ROK fields too many boats in a limited patrol area they increase the chances of collision with shipping traffic or other boats.
One Nuke boat may be able to do more than two diesel boats with fewer men more safely considering the size of the territory to be patrolled.
It is not an easily made decision.
The nuclear boats of today (on average) have about 100-110 personnel (based on British and U.S. subs); the current Type 214s of the ROK Navy have about 35-40 men, so you could in theory have 3 D/E subs for one nuclear sub in terms of manpower.
Given that the Korean Peninsula is not the biggest around, one would assume that they are looking at nuclear propulsion to carry a nuclear deterrent.