I partially disagree with you, that damping factor is marketing fluff, Benchmark has proven that damping factor does make a big difference in audio in terms of distortion test(with results on AP), which has been verified by a non-commercial third party. Anyway's, its notable that your source is yet another commercial tube amp seller, though the math is correct, explanation is missing something .
For the cable resistance argument, I say that for speakers and headphones, its a totally different ball game. I refer to "Audio Engineering Explained", on speakers, AC transformers in the loudspeaker wipe out the damping factor's effects on loudspeaker resonance.
This is however, notably untrue in the case of headphones as they do not have space to fit in transformers, which brings DF back into the headphone ball game. Damping factor's effects while subtle in some headphones(notably D2000) can be extremely audible in the case of Balanced armatures, for me it was the X10s and Shure 840s driven to the same volume from the ipod as compared to headphone jack out to the O2 before comparisons.
For anyone saying I'm such a "0 ohm" proponent, I do like to drive my headphones out from my receiver from time to time but I will stick with my O2 for resolution on the bass.
References,
http://www.benchmarkmedia.com/discuss/feedback/newsletter/2011/12/2/0-ohm-headphone-amplifier-sonic-advantages-low-impedance-headphone-amp
nwavguy's article on headphone impedance
Audio Engineering Explained for professional recording pg. 199
Sighted and single blind tests by my ears which are highly subjective