Richard M. Stallman wrote the original Editor MACroS for the TECO editor.
James A. Gosling implemented Emacs in C as a stand-alone editor, sometimes called "Gosmacs", and distributed it freely with no copyright notice. Gosmacs had an extention language called Mocklisp, which wasn't really a Lisp (it had no lists) but appeared similar.
RMS used Gosmacs to get started on GNU Emacs, which featured a "real" Lisp (close to Maclisp).
JAG sold the rights to Gosmacs to Unipress, who renamed it Unipress Emacs, sold it commercially, and stopped distribution of gosmacs and derivatives (like GNU Emacs).
Presumably it was around this time the interview with Bill Joy occurred.
RMS rewrote the part of GNU Emacs that was derived Gosmacs, mostly the display code.
One could guess that this experience is part of why the GNU project insists on signed copyright assignment or release forms for key utilities.