The point of the Korean's story is not that he learned English but rather how well he learned it and how fast. He estimated that he learned 30,000 words in 2 and a half years, and if you followed the link to the phone interview, the guy has no recognizable accent at all -- he sounds like he learned American English as a child, even though he didn't start really learning the language until he was 16. This is actually fairly remarkable.
First of all, that interview was after TWELVE years of living in the US, completing high school, completing college and working in the target language environment. Any highly motivated learner should have an excellent accent long before that.
A serious language learner can get to a very high level of proficiency in a couple years' full-time effort. Some of them have fun, too!
My sister-in-law's mom, who is originally from India, has lived in the U.S. longer than I've been alive, and yet she still has a strong accent. I'm not an expert, but in my experience it's extremely unusual for someone who learns a second language later than about 10 or 12 to ever completely get rid of a noticeable accent. Of course the first and second languages matter -- native Japanese and French speakers who learn English later in life rarely seem to loose their strong accents. Speakers of germanic languages seem to have it easier.
It would certainly be interesting to hear a sample of the Korean's speech from when he was 18 or 19. However, I think the reason he speaks English like he grew up in the U.S. is because he put a lot of specific effort into getting his pronunciation absolutely right.
A serious language learner can get to a very high level of proficiency in a couple years' full-time effort. Some of them have fun, too!
In general I agree with that. What the Korean did was clearly overkill for what just about anybody would ever need to do. However, I think you could employ the Korean's techniques to a lesser degree and still have fun learning a second language.