Actually those lines right there can be determined in quite a few ways. And even now days that line is till true when you think about it.
Just the line "My band had a singer", "They didn't need me" isnt really just about him rapping. It was more aimed at having two vocalist. Cos even back then Mike would sing a little.
And when you said about him proving people wrong, you cant tell me ATS didnt prove everyone wrong. The record company was against it, Some fans dont like it, but it still has proved those critics wrong by being one of their best selling albums so far.