Well, it's interesting on the human and historical side, especially the part that it was all self-taught, but on the technical side, it's not clear from this in

Well, it's interesting on the human and historical side, especially the part that it was all self-taught, but on the technical side, it's not clear from this interview that he's using GCR and how (https://en.wikipedia.org/wiki/Group_coded_recording#Apple https://en.wikipedia.org/wiki/Group_coded_recording#Apple) and that e.g. Commodore had a more efficient scheme that allowed a whole 170KB per side, at the same density.