There is also a fitting for AC cable (wiremold 5790B) but the instructions seem to imply that you use a "short section of raceway to the fitting" (from a box where the splice would be made?). I agree it is hard to find anything firm about how these are installed in the PDFs I have looked at.
I'm sure a careful installer could stuff 2 wirenuts in this "mouse" but I agree it is a violation of the box fill rule.
