That would be the source code posted to that sonic.net site. It was a mix of raw dumps of FORTH blocks and printouts converted to PDFs. It wasn't the full source code, and there weren't any design documents in there.
https://web.archive.org/web/20031218202130/http://www.sonic....
When you navigate those directory archives many of the files are missing. E.g.
https://web.archive.org/web/20030529230704/http://www.sonic....
Some design documents ARE there, e.g. "Specification of 3d display for starquest" : https://web.archive.org/web/20030719111039if_/http://www.son...