What does this mean?

I suspect that you are not missing anything, someone else may be though. I believe it takes 3 wire to be balanced.

From the ungodly, perverted vivisection on the Island of Dr. Moreau, could it be a balanced interconnect reterminated with RCA plugs?

ah Jameswei! You may have it!
I'll take a set.

Or, do you take the extra wire and plug it into the wall receptacle?

It simply means that one end of the cable weighs the same as the other end. If you coil it up and heave it into the woods, it will spiral gracefully into the void. Which is probably what you should do with such a confused cable. :-)
I'd try another seller, myself.
Still unbalanced,
Ed

Euro balanced interfaces sometimes use RCA's; 1 cable for inverting & another cable for non-inverting. Take a look at an Electrocompaniet component for an example of this approach.