As someone else said, pitching coaches are great, good, all right, and bad. Managers, GMs, and other components can also be measured as such...but you cannot declare without proof (in otherwords simply expressing an opinion) without some proof.
I can point to Johnny Sain (a friend of mine) who turned numerous journeymen and good pitchers into team aces. Look them up yourself, but Jim Bouton, Ralph Terry, Mudcat Grant, Dave Boswell, Jim Perry, Jim Kaat, Earl Wilson (a personal favorite), Mickey Lolich, Denny McLain, Wilber Wood, and Stan Bahnsen all became 20 game winners with Sain.
Orel Hershiser credits Dave Wallace with his turn around from a good to great pitcher. Nolan Ryan credits Billy Muffett with his teaching him to control his fastball, and Tom House for prolonging his career. Steve Carlton has credited Ray Ripplemeyer for his slider. Historians credit Norm Sherry with controlling Sandy Koufax incredible stuff. Larry Rothchild developed average major leaguer starters Kevin Brown, Al Leiter, and Robb Nen into star pitchers. Roger craig taught Jack Morris and others the split finger. Art Fowler turned fifth starter Ron Guidry into a 25 game winner. Dave Duncan turned several pitchers' careers around.
And there are lots of others..what proof have you got???