Don, you appear to be correct. I have not installed many C-Taps for GEC but have on occasion if the existing wire has been damaged or is too short for a panel replacement. The few I have used have been Burndy but I gave Al the equivalent Ilsco part number since the Burndy web site can be a pain to use. I actually called Burndy and they confirmed that these particular C-Taps are not listed for grounding. They do make a version listed for grounding but they must be installed with a hydraulic tool.
I guess I have never really understood why a connector that is listed to carry continuous current is not also listed for grounding.