Comparing disease outcomes at all is futile because they are incredibly sensitive to initial conditions. One sick person travelling in January made all the difference.
An immediate, thorough response makes all the difference.
Your "at the same time" statement is off by a few days and off by a few days makes a huge difference in outbreak severity. That's why you need to keep in mind that small numbers of infected people and seemingly short time intervals change the result radically.
I’m not sure what you’re talking about with airports. South Korea has had less incoming travel restrictions than the US.