Incidental language exposure through gaming is an awesome way to learn.
Note that he's starting from N2 Japanese, which is already a high level of Japanese proficiency (although it does not test writing/speaking at all, so it's very feasible to have N2 yet be terrible at conversation). He's not exactly learning hiragana from M:TG.
The M:TG competitions are giving him a framework to practice that conversation, which believe it or not can be hard to come by in Tokyo without deliberate effort (see 'expat bubble'). The vocab/grammar on the cards is mostly incidental to all that. If he was playing online M:TG in Japanese he wouldn't be getting anywhere near the payoff.
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.
That certainly is controversial. I don't think many people would consider anyone who is fluent to only be B2.
But I as far as I recall B2 is when you start seeing native people failing the exam without preparation with C2 becoming a legitimate challenge for native speakers.
I believe the same threshold exists in N2 but because it's so Kanji focused without much assessment of fluency.