I don't think this is correct. The headphones themselves certainly have an large effect on the size and quality of the soundstage. Obviously encoding binaural delay and other psychoacoustic features is important too. But Apple is far from the first to do this kind of precision stereo manipulation.
I don't know what to tell you. I've tested dozens of pairs of headphones and some have a clearly, obviously larger soundstage than others, with the same exact inputs. Others have a more "inside" feel. I think we must simply be talking about different things, because what I'm describing is a commonly cited property in headphone reviews and discussions.
