This section is just relaying a full accounting of the features of real-world localization that passive headphones aren’t emulating. It’s important to draw that distinction because certain headphones that implement Spatial Audio with head tracking are doing this. Speakers aren’t emulating this because they don’t have to, because they’re actually interacting with your ITF.
To be clear, there I’m just saying that the ITD/ILD cues that happen with sound sources in the real world (and with speakers) due to sound sources in the real world interacting with your ITF are not present in headphones. They are absolutely present in music, but faithfully presenting the stereo panning and timing of the music ≠ localization.
No one I know has claimed that headphones have some magical ability to create soundstage regardless of material being played back (even when fed a mono recording), and in fact stereo speakers can’t do that either!
I’ve seen plenty of people claim soundstage is some elusive acoustic quality separate from the music itself that cannot be measured through traditional means. Must just be in different crowds. Your crowd sounds more reasonable 
That’s where the whole discussion should start: headphones have soundstage because they reproduce the ILD that exists in almost all recordings. And then we get into all those other considerations about total loudness differences between instruments helping create the illusion of a depth axis, FR influences, “presence range”, HRTF matching/neutrality etc.
Eh, I don’t think ILD is actually how people are understanding/reporting soundstage though. Like people aren’t using terms like left and right to describe soundstage, but panning. To describe the former they’re using terms like width, depth, height, separation, etc. so IMO it makes sense to unpack why these things mostly don’t exist in headphones as a separate acoustic quality and why.
There are many - and I am one of them - who say the KSC75 presents them one of the widest most open soundstages available in headphones
I could see that, and I kind of agree! But I wonder if them being extremely light and not having a headband helps in a similar way, minimizing tactile sensations to make it less obvious you’re actually wearing a headphone 