I think more info. is needed as well, regarding effort, and details of the "mini bullpen".
My "guess" is that since they are throwing from an elevated mound, they are high intensity pitches. Kind of hard to keep same mechanics on the mound, yet throw with little intent. In addition, I don't think you accomplish much, if anything from the mound, at low intensity, or say less than 90%.
Stress from throwing from an elevated mound at 90% intensity is huge, especially at "lay back" position of the arm, before it launches into ball release. You don't see many strength development performed on repeated days, so I don't think you are building any stamina or strength. In fact, look at weight lifting, there is always sufficient time built in, between working body parts, to allow to heal....that is how you build strength.
Edit to add: I don't think you accomplish any fine tuning or "feel" at lower than 90% effort in pitching. You may be gaining feel, or fine tuning for pitching at 70%, but that does not translate when you are now on the mound throwing at 90%+. Same for developing off speed, or grips...yes you are learning those at 70% effort, but you cannot expect the same result at game intensity.