I think this generally is not true, although for some players it certainly does seem to be and likely means they were so good at a young age that they could have played at a high level, but for some social reason or whatever the team was more confident starting them lower in the organization. Hamels vaulted through the org with very few actual IP, because of injuries. He did physically mature during that time, but I suspect he could have thrived at a higher level than he was placed.
On the other side, we've seen many good control/little stuff pitchers over the years who thrived in the lower minors and couldn't succeed higher up the line.
I think part of you conclusion is illusion: as the minor league season progresses, the leagues become weaker, because the best players are promoted out of the league and many of the regular players are out because of injury or promoted just to fill holes farther up the ladder. So the guy who gets promoted in July and does well very well may not have done nearly so well if he started the year at that level, before the ripple of player transactions weakened the league.
I also think pitchers make more mistakes at the lower levels and that a guy with less than good hit/pitch recognition skills, but who has plus power, can look very good hitting mistakes at the lower levels.
I have frequently observed over the years that guys who do well in Reading don't necessarily do so well when promoted to Allentown to start the next season. Hoskins really isn't normal in that regard.
