I think that when he says "Immersion doesn't work" he really means that "Immersion alone doesn't work". Of course immersion is helpful, but it doesn't produce results by itself (at least not in Korean; maybe a language with more cognates with English would benefit more from immersion alone). Non-immersed learners sometimes use the prospect of immersion as an excuse, along the lines of "Once I'm immersed in the language, it will be much easier to really study effectively, so it'll be a more efficient use of my time to take the studying easier now."

I've made both of these mistakes myself (overestimating the value of immersion and slacking off study when not immersed). When I first went to Korea, it was a huge ego boost since it felt amazing to actually be using the language. Because of this ego boost, I felt that "book-study" was suddenly beneath me and stopped. After a few months of immersion, though, I realized I was learning more slowly than just going through flash cards, so I picked them up again. Even though immersion was useful for me, it was really only effective as a compliment to other study.