Exactly. The guys whose job it is to know such things see some thing(s) he needs to work on. There have been references on these boards about some issues with breaking balls. I have no idea if that's what it is or not, but let's suppose it is. If he's having some trouble with them against AA pitchers, major league pitchers are going to eat him alive with them. While Devers may be destroying fastballs, we have to remember that AA pitchers are every bit in the same development mode that Devers is, so their organizations may be having them work on commanding fastballs rather than tossing breaking pitch after breaking pitch just to get a certain hitter out. Big league pitchers are not going to be under any such requirements.
As you point out, he's had 250 or so ABs above A ball. Benintendi at that same age was still in high A ball. So was Betts. JBJ was still in college. Xander got to Boston, but his first full season (2014 when he was 21) was a struggle.