Maybe, you could be right. Certainly Italy and Spain have already exceeded South Korea's cases per capita, as well as deaths per capita: Italy, Spain, and South Korea have respectively 1809, 335, and 75 deaths†, with populations of 60.3 million, 46.7 million, and 51.7 million, for rates of 3.0, 0.7, and 0.145 per 100,000 population to date. So it's implausible that South Korea's dramatic improvement is due to a much larger number of subclinical cases saturating the population and preventing further spread, and I understand that their testing is quite comprehensive as well, so it probably isn't due to underreporting either (which in any case is less likely for death rates); if that's true, it would seem that the only plausible conclusion is that containment is working in South Korea.

I think it's reasonable to expect that similar containment measures will be put in place in most countries soon, though, so perhaps it's not unreasonable to take them into account when trying to estimate future growth rates.

† https://en.wikipedia.org/w/index.php?title=Template:2019%E2%...