> I met this somebody in Japan. If I remember correctly, he spend a summer "doing" RTK, then took 1 semester Japanese at his home university, went on exchange to Japan for two semesters, and after finishing his first semester abroad he passed JLPT 2 (not N2 - this was before they added the N)

While I certainly believe your story, I hope that you know he is an extreme outlier with super-human level of memorization and recall. Tiny question: Do you know if his uni with in the countryside or a big city? The people whom I have met that gained fluency the fastest (normies here, no superhumans, please!) all had significant time lived in the countryside, so they had an immersive language learning experience.