Geographically yes, it is much easier for South Korea to isolate itself.
Based on the extreme density of South Korea's population, no. South Carolina has one tenth South Korea's population.
South Korea's population density is much greater than what we have outside of our top two or three cities.
50 million people on a relatively small peninsula means that if the Koreans hadn't reacted quickly to the virus it would have been uncontrollable within a matter of days.
If New York City had been as decisive and well prepared as South Korea there is every reason to believe they'd have fared much better.