I can't say exactly where in the OSHA documentation it says this (again, it's been a few years) but they don't have to hit the 5000# capacity when you provide other means for energy dissipation. The spring (or whatever is in those anchors) extends the distance (and time) for deceleration, thus reducing the force on the roof/connection. These are tested assemblies that comply with other aspects of the OSHA requirements.
The 5000# rule comes into play when you have a rigid element that you're tying off to. Think your typical post welded to an embed plate on a concrete roof. That has to be designed for 5000# because it has no other means for dissipating the energy.
I did not end up recommending using those anchors. It was mostly cost related reasons. We designed a rigid element for the 5000# requirement.
I'm not saying the Miller product is bad. It's tested and I'm sure works just fine for protecting anyone tied off to it and definitely has applications where it works best (like existing structures). I suggest getting in touch with one of their reps and learning the pros/cons of their product so you can, at the very least, be informed for the client and make a good recommendation.
