Statements that an audio cable or component can or cannot "pass a square wave" are often bandied about in relation to audio, but without specific QUANTITATIVE elaboration they are meaningless and potentially misleading.
Depending on the frequency of the square wave, on the transition times between its higher voltage and lower voltage states (i.e., its risetimes and falltimes), on the time-scale at which it is observed, on the degree of degradation of the square wave that is considered to be the threshold between passing it and not passing it, and on whether the degree of degradation corresponding to that threshold has any likelihood of being audibly significant, either every cable in the known universe or no cables in the known universe or any number of cables in between can be considered to be able to "pass a square wave."
Since in this case the person making the statement was an engineer and audio designer he presumably and hopefully was basing his statement on parameters that are meaningful in the context of audio. But even if we make that assumption, the degree of degradation of the waveform corresponding to the threshold between passing and not passing a square wave will be debatable, with it being possible to make a technically plausible case in support of a wide range of characteristics.
Finally, the ability of a cable to "pass a square wave" may be significantly dependent on the output impedance of the particular component which is driving it, to a greater or lesser degree depending, again, on the various parameters I've referred to above. Although that possibility is most likely to be applicable to interconnect cables, rather than speaker cables which are being discussed here.
So I would take any such statements with grains of salt liberally applied.
Regards,
-- Al
