Strength is a plus, no doubt, but.........Swing Technique is more important than strength.
Good technique, or swinging inside the ball is what allows you to lay off pitches and swing later. Quick hands, not strong hands, allow you to lay off pitches until late and then when you combine that with a nice short, or inside swing you get a bat that stays in the power zone longer. The power of the swing comes from the legs and hips. But it's not just brute force. You have to have good technique and timing when you bring those hips and legs into the swing.
I'd say most of Bonds improvements in hitting and slugging percentage have come from him improving his swing mechanics over the years. Plus, 11 years of seeing pitches has got to help you become a better contact hitter, you at least get more discerning in the pitches you swing at.
It's a simpleton's arguement to lay it at the feet of one thing (drugs). If he is taking roids, then I think it's stupid, because it doesn't help your hitting as much as working on your mechanics.