I've never seen Johnson fight before but from what I hear, he doesn't really fight this way. I think he was so intimidated by Vitali that he just wanted to last as opposed to trying to win the fight.
See your problem here is that you're operating on the erroneous assumption that Kevin actually does possess an "offensive arsenal," as you call it. This is not so. In Kevin's case his best offensive weapon always has been, currently is, and will continue to be his mouth. Period. Anything else is negligible at best.
He developed the right hand a bit, he used it quite a lot in the Bruce Seldon fight. There wasn't much more apart from that. In fact, in that fight he got out-jabbed by Seldon early and was hurt on occasion.
No. Other than his last couple fights, he's always just been a guy who jabs his opponent to a wide decision.