It's all interesting but I think merely serves to limit how good of a comp Kelenic is for Julien.
The Twins had a problem with lefty hitters against lefty pitchers in 2023. Second worst OPS among all major league teams - Kepler was acceptable, as was Gallo (!) and Gordon (!!), all the others were below .500 (!!!). It may not be simply a matter of lack of opportunity, but somehow they're not being coached to succeed, in which case force-feeding isn't the answer and better technique (whatever that may be) is.
Wallner for instance appears to be a fearless batter - he gets a lot of HBP which I assume means he won't back down, and I frequently bring up the AFL game Chief and I watched where he took a pitch off his jaw and he was back in the lineup a few games later and hit like nothing had happened. A tough guy. I don't believe he's flinching when the ball comes from that direction. And yet in his short MLB career his Right/Left split on OPS is .949/.442. That's almost an inconceivable split. It's still small sample size, and he didn't exhibit this problem coming up in the minors, so I don't claim to know the answers.
But improvement is mandatory, Priority #1, Wallner and all the other lefties, because 13-man rosters aren't enough to platoon everybody. I don't think Rocco substituting or not substituting is the root cause.