For me, the only reason I'd wonder if he was really natural was the routine that he supposedly used. It was a TOUGH full body routine with several exercises per body part (instead of the standard 1 per body part full body routine) and it appears he wasn't using light weights. Sure it was just 3 days but I would think it would be hard to recover and would lead to overtraining rather quickly.
From his book building the Classic physique.
Upright rowing 3x8-12
Press behind neck 3x8-12
Bent over laterals 3x8-12
barbell Bench press 3x8-12
Dumbbell incline press 3x8-12
Flat bench flys 3x8-12
Chins or pulldown behind neck 3x8-12
Low pulley rows 3x8-12
One arm Dumbbell rows 3x8-12
Standing Barbell curls 3x8-12
Incline Dumbbell curls 3x8-12
High pulley bench curls 3x8-12
Tricep pushdowns 3x8-12
Tricep extensions with Dumbbells 3x8-12
one arm cross over or Dumbbell cross faces 3x8-12
Half squats 3x8-12
Barbell hack lifts 3x8-12
front squats 3x8-12
Buddy assisted leg curl 3x8-12
calf raises(leg press machine) 1x20
Good mornings 3x8-12Partner assisted neck flexation 2x15