EL and/or Kiley have talked about how all K rates at the AA/AAA level aren't created equally.
A guy like Bellinger had a high K rate coupled with a high BB rate. This suggested he was working counts to get pitches to drive, and his high K and BB rates were a function of deep counts. That skill set tends to translate to the MLB level well.
A guy like Riley also had a pretty high BB rate to go along with the high K rate, but his skill set didn't translate to the MLB immediately (outside the insane luck streak). This tells me simply looking at K/BB rates isn't good enough...someone actually needs to see the approach, as it was clear Riley was guessing most of the time at the MLB level and looked terrible when he guessed wrong.
If we had MiLB plate discipline numbers like Z-Contact% and O-Swing% we could determine why the K rates are what they are, much like I did very quickly with Riley to determine there were serious red flags with his offensive profile, even during the luck streak. Unfortunately, we don't, so all we can do is guess who can really hit and who is simply feasting on MiLB pitchers unable to execute pitch sequences. I'm guessing the Braves have this data, and are making decisions accordingly.