I doubt it had anything to do with the way he trained it, his back was just a genetically weak. Nothing much can be done.
If you ever watched vids of him train, especially back (like lat pulldowns) he would just pull the weight down real fast, with no squeeze---just up and down. Also, his back would "open up" in the FDB pose or variation of that, but not when doing a FLS or RLS pose. So, it tells you something about his training and posing.
How people train tends to be unique to them. Dillett's workouts were famous for being uninspired and he would have trained his back in a similar way he would have trained other body parts. However, while those other parts responded superbly, his back remained stagnant. That's a clear sign of a genetic issue. He obviously would have been aware that his back was holding him back so I'm sure he would have tried different routines, exercises, rep ranges...etc to trigger a response but it remained a weakness.
With a little more thickness, and opening up both sides in a proportionate manner, his back wouldn't have been a liability here:
Same condition as Jason Genova............
Poor poser, poor training habits, and insane genetics. What a waste. I heard he was a good poser outside of contests, but when he would step on stage, he'd just freeze up.
/ wtf being attacked by schmoes...[i know what really happened]
Problem is, you can't necessarily train back like other bodyparts. Just look at how Bertil Fox trained. He was completely two different bodybuilders, one from the front, and one from the back. His odd training didn't work that well from the front. And are really sure you think he would have tried different routines? The man couldn't pose for the life of him. That takes practice. And it seem like he didn't do that. he didn't care. Plus, he had shoulder injuries.
This is what I would suspect........no mind/muscle connection.Very hard to establish for most people when it comes to training the upper back/lats areas.