Lurker0
Headphoneus Supremus
Impedance only matters for waveguides, i.e. conductors with a length comparable with the signal wave length. S/PDIF transmits signals with maximum frequency of 12MHz (for 192kHz sample rate, 32 bits, stereo), meaning, with wave length more than 15 meters. So, for a cable less than 1 meter, the impedance does not matter at all.