Perhaps he's saying that a mental reset / change of scenery can be helpful to someone.
Perhaps he's saying that it is easier to work on mechanical changes in an environment where the games don't "count" as much and you aren't feeling like you are letting your teammates down if you struggle. Maybe development doesn't completely trump winning at AAA, but the ratio of the importance of the two is skewed very differently than it is at the big league level.
There's a luxury of being able to experiment and tinker with things at AAA without any pressure that if something doesn't work, or takes a long time to work, you will be sent down.
A college player is always a safer pick than a high school player, but often the HS players turn out to be the future elite MVP types. High school higher risk, higher reward. It's where you find your Gunnar Henderson's, Bobby Witts, Jackson Hollidays, Manny Machado, etc. A couple years ago I looked at the All star game roster and 70% were either international (very young) or drafted out of HS