Either they assumed you would use a sub with speaker level inputs or most likely the wire wasn't necessarily intended for a sub. You just need one cable for the sub and it can be a premade cable with rca jacks on each end or you can use coax and crimp on your own rca plugs if there is a spare coax coming out of the wall near your sub.
A typical pre-wire for a new home (in this area at least) includes one cat5 and one coax to each room and additional (seemingly random
) sets of 16/4 all around the house. It's just wire and can be used for anything - security system, speakers, lighting, etc. My particular house has 16/2 wire for the speakers but additional runs of 16/4 which are not being utilized at the moment.