I believe it is improper to apply the box fill rule here....assuming the component is part of a listed system. As such, a determination was made that there was adequate space.
A similar situation occurs with every bath fan...the wiring space is a lot less than your usual box.
I actually think that these things are treated a 'appliances,' as their application is so narrow.
FWIW, I think you'll find the elbows in these systems are also somewhat tighter than we are allowed to make. And I defy anyone to pull wire through a 'completed raceway' of this stuff....you have to piece it together as you go.