He's been rapping since the group started...I mean way back, when they had a little 4 track Demo, and went by a different name. Their lead singer was a different guy at the time, but Mike was still rapping..
He even said, as a kid the first rap video he ever saw was Biz Markies "Vapors", and he thought it was dope. He just grew listening to more than one genre, and liked it all...
Just because he's in a rock band, and likes rock music doesn't mean he doesn't know hip hop, and that you can't take him seriously as a rapper.