Yes it is sort of the length but only as far as having the correct 75 ohm impedance. SPDIF is very cable sensitive, much more than RCA
This is quite an old article but still very valid http://www.tnt-audio.com/accessories/digitalcables_e.html Chris Sommovigo is one of the top cable designers for companies such as Stereovox and Black Cat
Once upon a time, think it was Wavelength wrote an article about spdif cable length. Without boring people too much, the point was short=bad. Which would suggest no cable=real bad ;) Came down to the time it took for the reflection to reach the spdif output. After Xft, the reflection doesn't affect its own output. I believe its was 1.5m for 16.44. 24/192 would need less length.