It is not semantics -- the cables are directional at very high frequencies, due to things like surface composition, grain and void structure of the conductors and dielectrics, irregularities in cable and PCB construction, and all those tiny little things that matter a great deal when you hit mW/mmW frequencies (and not at all at audio frequencies). But note we are not talking about diode sort of directionality, where current only flows one way, but differences in the impedance that charge carriers and magnetic fields see traveling in one direction vs. the other.
For those not following, S- (scattering-) parameters are used to measure the levels of (usually) RF signals, and Sxy indicates the signal at port "x" from port "y". Thus for an amplifier or cable with port 1 being the input and port 2 the output, S21 indicates the signal at port 2 (output) from port 1 (input), and S12 the signal at port 1 as driven into port 2. For an amplifier, S21 is the gain of the amplifier. For an ideal cable S21 = S12, meaning the signal is the same at either end when driven from the other end, but at very high frequencies they are not equal. The cable is directional in the sense that the transfer function from 1 to 2 is not the same as the transfer function form 2 to 1.
HTH - Don