Yes pitch counts matter more than IP. Horton;s situation is better than the IP because he's been throwing strikes and been effective/efficient and avoiding walks and K's. I'm also not arguing his treatment is as bad as Mark Prior.
But they have 2 months to pitch, and they need to be at their strongest in the playoffs, not running on fumes. It appears obvious the Cubs have no plans for any significant rest for either Boyd or Horton unless they show fatigue (which Boyd could be?) or get injured. Were Prior and Wood showing fatigue in 2003? Who knows but it didn't seem that way from what I remember. Comparing pitch counts from 20 years ago to now is also not 1:1.
Some preventative caution doesn't seem unreasonable at all. When the load on a part of the body is too much it can cause injury in an instant, it doesn't always come with warnings. The data shows pitching injuries dramatically increased in 2021 and 2022 because of limited usage in 2020 and have been trending down every year since.
This is also an org that just traded for a SP with declining velo red flags and then immediately threw him on the mound and got immediately hurt. 2 years ago they also pitched their oft-injured closer into the ground because they were desperately short on pitching talent, which doesn't seem much different this year with the rotation. They also threw Steele back on the mound in late Sept last yr for meaningless games after a couple weeks out with an elbow injury. instead of shutting him down. This org doesn't have a clue and continually ignores red flags and shouldn't be given the benefit of the doubt.