Everything else is either too slow or doesn't have the up-to-date gamedev ecosystem.
And you'd have to be insane to write a game these days in C. It's just not a productive enough language for that. Whatever little speed you give up for Java pays off many times over in speed of development, debuggability, etc.
Would be interesting to know if he is using Scala for any parts of it though.
Of course if you like Java, use Java.
More impressively if you use C# you can release cross platform using mono.
With the depth of this game and the FPS type elements though I'm skeptical whether it will make a good iPhone game or not though.