Zobrist opening spoke about how Jaime Cevallos completely rebuilt his swing and approach, which ultimately lead to massive improvements. I mean I guess it could be the plasebo effect to some extent and he was going to become a stud hitter regardless, but personally, I'm certainly buying the swing change was the primary factor. Same with Muncy man...he openly talked about how him and his dad rebuild his swing in the 2016 offseason in one last effort to be a ML ball player.
https://www.draysbay.com/2009/4/23/849233/cracking-the-zobrist-code-what-is
Also - why are you suggesting JD and Muncy were elite level performers in the upper minors? Max Muncy posted wRC+'s of 102 and 108 in AAA before the swing change. JD posted wRC+'s of 103 and 94 before making changes. These players were s***.