Then you haven't been watching him pitch then. His slider is brutal and what people actually strikeout on. The "only one pitch" argument is just wrong since Chapman only uses 2 pitches because he is in the bullpen. He has more but when you only throw 20-35 pitches in a game, it doesn't make since to prolong AB's with change-ups and curveballs.
As a starter, Chapman would use a 98 mph fastball which is still 5-6 mph faster than most starters. Hitters will still have trouble catching up with it.