Arnolds recommendation sounds like he assumes everyone has the exact same genetic makeup of fiber types. Some people have more fast twitch than slow twitch in a lot of different areas.
This is the same Arnold that thinks close grip benches stimulate the "inner chest" while wide grip stimulates the "outter chest".
Bench presses are not "power exercises" and neither are squats.Arnold also recommends going heavier than that on certain exercises, especially the power exercises such as bench and squats from time to time. These ideal rep ranges are generally what it takes to fatique all of the muscle fibers in that group
Olympic lifts are power exercises.
Power is work per unit of time.
Myofibrillar hypertrophy is the thickest form and is stimulates better with low reps.