I'm not sure what you're referring to since the files T.C. Lee posted on that geocities site are the only design documents he (or anyone else) ever released, and those are preserved in the SFFiles.zip found at the oocities mirror linked from the github there. That zip contained only a partial dump of some source code, while that sonic.net page contains a more complete copy (but lacks any kind of design documents whatsoever). Was there something else you were referring to?

I seem to recall browsing what appeared to be the complete source back in the day. I put in a bit of an effort to get it compiling, but it was only released as poorly scanned printouts of the source code and OCR wasn't so good then so the project was bigger than I hoped.