I still don't understand how that translates into headphones not presenting decay or all headphones presenting decay the same.
Different headphones do not all present the source material the same way due to a variety of factors. Regardless of what specific the cause behind hearing more decay or less decay on different headphones, if there is variability in the presentation of attack/decay, then you can talk about a headphones' ability to reproduce attack/decay.
So you are saying that decay, defined as the reverberation a fading note, is always just recorded into the music. However, if you hear the decay being longer or shorter on different headphones, isn't it a valid sound quality to discuss when comparing headphones? You can even remove this confounding variable by playing a test tone that has not been engineered with additional decay, since that actual note will decay after it is produced by your headphones & you can judge how well your headphones present decay.
There are "Cumulative Spectral Decay" graphs that measure the differences of the decay of a single note in different headphones.
edit: It's like saying there is no such thing as sound stage because the sound stage in determined by the source, but people can obviously hear differences in how headphones present the sound stage present in the source.
He's saying the decay added by headphones, even "slow" ones, is insignificant compared to the amount of decay already added to the recording from the live performance or from studio magic. He's also saying the differences in "speed" between headphones is not because of delay, but because of differences in the volume of frequencies. Particularly, IMO, differences in bass and lower midrange which will cause masking of "detail" in the upper midrange and treble.
I tend to agree, but don't know enough to take a strong stance for it. Still, it makes a lot of sense when you think about it. Isn't it convenient that all the audiophile detail darlings like the HD800 have emphasized treble? Bright and detailed always go hand in hand in subjective impressions.
But then, so do cables and improvements