Someone carelessly pucks at the strings of a guitar, the worst that may happen is the string will snap.
Someone carelessly pulls the trigger of a gun, the worst that may happen is that someone dies.
Comparing firearms to guitars is "silly" to put it in the politest way possible.
Jeff Cooper made a lot of dubious statements based solely upon personal opinion and this is one of them.
You don't need to be an expert marksman to use a firearm effectively, you don't even have to be proficient with a firearm, you just have to be competent enough with it to know how it works so that when you pull the trigger it goes bang.
Furthermore, the average armed confrontation typically occurs within 7 yards, at that distance it's fairly difficult to miss, and believe it or not your ability to consistently hit stationary or otherwise predictably moving targets at the range isn't necessarily going to make you any better prepared for an armed confrontation than the novice who only possesses a basic understanding of how to operate their firearm, use the sights, and hit their target.
Disagree all you like, but that doesn't change the fact that your proficiency in the well-lit, safe, calm, recreational environment that is the shooting range does not translate well to a dimly lit parking lot at night or your bedroom at 3:00am when you are unexpectedly accosted/attacked and your senses are dull from surprise, fear, and other factors such as the shock from suddenly/unexpectedly transitioning from a very relaxed state such as sleep or boredom from the monotony of your everyday routine.