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.

As far as games go, tabletop RPGs are probably better than MTG because they are all about talking. But nothing beats doing what you enjoy doing, and if what you enjoy is MTG, then MTG is the best.