Where they say so? The initial paragraph says:
s7 is a Scheme interpreter intended as an extension language for other applications.
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s7 is an extension language of Snd and sndlib ([snd](http://ccrma.stanford.edu/software/snd/index.html)), Rick Taube's Common Music (commonmusic at sourceforge), Kjetil Matheussen's Radium music editor, and Iain Duncan's Scheme for Max (or Pd)
[...]
I cannot find the fact that it was “designed for…”. My experience is that it is a very good software which is daily maintained (see the git log here https://cm-gitlab.stanford.edu/bil/s7/commits/master) by a very competent programmer. The fact that people do not know it does not mean that it is not good or unfit for other purposes. See for example this experience in embedding it in a game setting : https://carloscarrasco.com/love-letter-to-s7-scheme/
Personally I think it is even better than Guile, less elegant maybe, but with a lot of cool features and it is programmed with a lot of taste and love, what do you want more from a piece of software?