Contraversial opinion perhaps, I don't think the cards or the game itself took him to fluency. 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…
Also N2, even N1, is generally not remotely fluent. Plenty of Chinese can pass N1 and still failed to have a conversation. Further, it's easy to pass N2 and/or N1 and still not be able to read most novels or listen to most movies when they get to things like legal proceedings, military strategy, science. All things that people can easily do when actually fluent
Source: live and work in Japan