They don't mention who the competitor is. But I've asked them and if I ever get a response, I'll let you know.
They seem to indicate that the graph is suspect below 150Hz due to so-called anomalies. No speaker cable that is sized properly would perform in that drastic way.
But to answer your question, the voltage that appears at the speaker terminals does indeed depend on frequency. Cables are characterized by resistance, capacitance and inductance. Resistance attenuates signals by the same amount regardless of frequency. But the resistance (called reactance) developed as a result of the capacitance and inductance varies with frequency.
At higher frequencies, the voltage at the speaker terminals does indeed drop by as much as a dB for "normal" wire runs due to the combined effects of resistance and reactance. But at above 15KHz, the minimum detectable change for humans with good hearing is somewhere around a couple of dB. So I don't believe anyone can hear the difference with "normal" wire runs.
