They are referring to the amount of clearance between the lens rear element and the sensor. The C mount flange focal distance is 17.52mm so both of these lenses have rear elements that stick into the camera body.
This fact is most relevant to SLR cameras that have a mirror which moves out of the way when a photo is taken, a lens that sticks into the camera body could foul the mirror. The C mount is exclusively (to my knowledge) used with cine cameras, microscopes and industrial/surveillance cameras which have no reflex mirror.
Having a lens that is heavily recessed into the camera body can make changing lenses slightly more difficult, but other than that the rear focal length is largely of academic interest as far as I can tell.