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.
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.