People are rightly complaining about the discrimination this involves, but if you look at the numbers, it doesn't make much sense as an antivirus measure.
The benefit will be marginal at best, and quite likely counter productive because visiting a clinic carries an infection risk

Korea has had 316 new COVID19 cases over the last 15 days. Of them 85 were 'imported', 34 of which were foreign nationals.
But the measures only cover re-entries of registered residents. So many of these 34 would not have been stopped by the new measures.

I don't know how many of those 34 were returning residents, but according to immigration, only half of 34k foreigners entering Korea in April were on relevant visas. 7,000 were students, and several would have been on newly issued visas, so maybe 10 people would have been covered

The new rules don't require a test. It's a doctors certification that you have no symptoms. But Korea has required all flights to check passengers before boarding since March. How many of those 10 patients would have had a fever at the doctors but not on the day they flew? One?

I suppose one is not zero, and would be two over a month.
But there's a cost. Everyone covered has to go to a clinic or hospital, during a pandemic. The incubation period means anyone infected there won't become symptomatic before they travel to Korea.

If 5,000 people covered by the policy enter Korea a month, how many of them will be infected at the clinics? It's quite likely to be more than one, in which case the net effect will be to increase imported cases.