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.
More importantly, being immersed provides an emotional reward for learning. If I learn these words, I can express these ideas and talk to people! This is different from just learning at school.
Edit: One thing I find odd about your post is the emphasis on confidence. It sounds like a correlation/causation fallacy.
I suppose the gripe I have is not "wrong; humans can learn languages by immersion!" but "why the societal crosslink between deliberate learning, and not-fun?".
Is there no way to learn 30,000 words by any motivation other than fearing for your life from a disappointed father?
The line blurs more when he says "Immersion doesn't work" then describes his learning method which is ... (wait for it) ... completely submerging himself in the language he is learning for hours and hours, reading it, writing it, speaking it, paying attention to it - all in a country and in a school where everyone spoke it ... immersing himself in it.
I cite: http://www.fluentin3months.com/studying-will-never-help/
"""Studying will never help you speak a language, but (as long as you do it right) studying will help you speak a language better.
Most people don’t see the difference here. That one crucial word changes absolutely everything you need to take into consideration.
If you already speak but your conjugations aren’t great or you need to quickly increase your store of vocabulary about a specific topic, then by all means study. Need to pass a test in school? Sure, study for it. When the goal is to pass a test or improve your grasp on something specific, then study is the way to go.
But if you don’t speak the language confidently right now, then it’s time someone broke this news to you: studying is not the way to get this confidence!"""
(But they are aiming at slightly different targets - Benny aims to be conversationally fluent in 3 months, The Korean to gain college level mastery in two years).