I'm not sure they are the same editor.  The C program called s is part of Webb Miller's Software Tools Sampler and it's a full editor, not a front end [see: Webb Miller's S Tiny VI editor ].  By design, it is mostly vi compatible.  This paper from Chris Feazer seems to be a front end for SWT editor, and was original done on the PDP-10 [which makes sense] and seems to be contemporary to the vi work at UCB and in fact called back to the SWT editor (which was >>modeled<< on UNIX ed), as opposed to ex/vi which are extensions to the v6 implementation of the UNIX ed editor.