Probably the social contact.
I mean N2 (JLPT levels run from N5 competent beginner to N1). Is really quite advanced.
Being N2 is far further than many will ever make it into learning Japanese. To arrive at N2 is very impressive. I think typically N3 is minimum for work on Japan (outside of lower end jobs or things like TEFL).
But JLPT is heavy on theory and light on practice.
It makes sense to me that someone with very little practice but pretty advanced grammar, vocabulary (including Kanji and spelling). Would rapidly pick up fluency if they got a reason to speak.
Not to discount the MtG effect but N2 is approximately CEFR B2 which is fluent. It's just that N2 doesn't assess fluency meaning you can get there with near zero confidence in conversational Japanese.