There's tons of bad instructors out in the world. There are also tons of kids who won't/can't listen to instructions and perform. Seeing a kid pitch without observing the instruction isn't really a good example of gathering empirical data to determine what the problem is.
For long distance, "power" overhand throwing, the throwing shoulder starts low, and goes up, reaching its apex just before release. For power underhand throwing, the throwing shoulder starts high (at 9), and then goes low just before release.
It is part of the whole chain...the shoulder leads the upper arm which leads the forearm.
What I tell the girls I work with is to "soften" the back shoulder slightly and let it follow the arm through the finish.
I've found if you tell them to "dip" the back shoulder they end up exaggerating the "dip" and they end up with the hand "falling" out of the bottom of the arm circle leading to timing, leaning and balance issues.