I believe this rules is related to 2-door pickups that do not incorporate full-size "normal" rear seats. Please check back later and I will have a definitive answer. You may have discovered a rule that needs to "clarified" to prevent confusion.
This was my interpretation as well, that's why I asked Jereme Creamer for clarification. His thought was that "jump seats" are those seats that are typical in a S10. When those seats fold up they fold up into a little pocket that is molded into the plastic. Seats in a Full size Silverado or the like just fold up against the rear. The jump seats in S10s fold up and get out of the way, the seats in a silverado just kinda compact they don't get out of the way.
With that said I think there needs to be more thought in classification for Full Size Trucks, with or without rear seats. When the S10 jump seats fold up they fold up into the stock panels which make it almost like there not even installed. That same cant be done for full size trucks.