This is largely true. Though, as with anything in the body, the entire system reacts. Bones will change density, the circulatory system also changes in relation to the increased mass, the cerebellum will re-wire due to the increased muscle mass and usage cases, the reproductive systems may change depending on many factors, the pancreas will change it's output of glycogen, etc.

When it comes down to building muscle mass and 'dexterity', the physiology is fairly well understood. For most athletes today the major issues are in repair/strain and consistency. AKA: Sleep well, eat well, train well, in that order. It is theorized that a lot of the recent gains in MLB pitching are due to improved tendon repair chemical therapies and traditional steroids that focus on bulk (no data here, I think I read it on 538 a few years back)