Finally, a non-umpiring thread in which I can contribute--here's why...I've seen two pitchers almost identically situated to Slider's kid; I umpired the last two SEC pitchers of the year when they were in HS (anecdotally, one went to HS with my now-wife.)
It may not be an issue about what the staff wants the pitcher to throw; it could be a matter of what the catcher can handle. When there's an obvious talent on the mound that is that much better than everyone else, the catcher's talent often can't keep up. That fastball at the top of the zone ends up looking a ball high because the catcher tends to pull it up, that fastball on the edge looks more outside because the velocity pushes the mitt out, etc. I've noticed that the inside and bottom edges are the ones that these catchers can handle because the extra movement of the glove isn't as drastic and isn't as much moving out of the zone as it is simply moving it in a slightly different part of the zone.