The number is from a press release, so unless there was a mistake in it, I don't see why they would intentionally under-quote it, as they want to encourage developers to use their platform.
Also note this is just for 3rd party developers at Mobage. Mobage's focus has been more on its own titles then other 3rd parties.
Mixi and Gree are Japanese SNS that support 3rd party apps. In particular, third party developers have been having a lot of success with Mixi.
So yeah, there is lots more money to be made in SNS applications in Japan, but this article was just on Mobage.