Different topic but not big Chapman fan, I think he has a flat fastball.
Fastball needs to have some movement, call it whatever you want. If it is flat it is going to be hit hard.
Agree that a flat fastball will get rocked, especially if it is down the middle of the plate, but if a fastball has some movement on it, especially late movement, and a pitcher can hit their spots, it can be a very effective pitch. My DD has a screwball "tail" on her 2-seam fastball and it drives me crazy when coaches refuse to call it because "we don't throw fastballs"...