It doesn't say that either. It says he sings the Poison tracks.
And it never said he isn't a boss. He may be a boss of a different type unlike Battle Mode with Slash. Like hitting a certain score/rating (this is a legend after all..whooo).
From a source like this do you expect all the complete info? I'm waiting for Neversofts take.
Exactly the point, he SINGS. It's not a guitar battle if he doesn't play guitar. It's called a boss battle because you go against a boss utilizing the new battle mode, Battle mode will not work unless the opposing boss plays guitar, completely destroying the possibility of him being a boss. And on top of all this, Activision even said there are three boss battles, one of them being slash, there are 2 other bosses in the game. It's three bosses, three boss battles, not two bosses, three boss battles