I don't see the absolute here. You can say the same about a manager then, or a teacher. Yet we all know that there are good managers and bad, good teachers and bad. I'm guessing most of us can give examples of both. I don't think it's true because it doesn't take into account things like personality. Some people teach a certain way. Some people learn a certain way. Sometimes those ways are incompatible. Likewise, some coaches see things that some don't. I don't think it's an accident that baseball people will say that "Don Cooper is one of the best pitching coaches in baseball". I don't think it's an accident when players say that their coach helped them make an adjustment that made them better. It happens. Coaches have influence, and some can see things that others cannot.