Well the fact that Shigeru Miyamoto also says its not a game works for me. Can you tell me why GTA series isn't a game? Cause I explained why Wii Music isn't a game. Also don't read too much into the tl;dr tag, jeeze.
Real gaming as you put it, is as its always been. An activity that requires some skill, within a set of rules. Some games are really really easy, some are really really hard. Wii Music does not require a skill set. I didn't say it couldn't be fun, I didn't say it wouldn't be entertaining for some or even alot of people. But it is not a game.