It's not clear that South Korea does have the pandemic under control - their new cases actually went up slightly in the last day, and there's been worrying talk of small clusters that have the authorities there concerned. The reason they're a problem is that smaller clusters are incredibly hard to find with testing and tracking, so if they're finding some there are probably more undetected ones out there multiplying.
if they find them quick enough they can still stop them from going exponential, but to do that you have to test more people faster. failing that they'll be forced to go into lockdown in more areas.
