As someone who has been using lisp for over 20 years, kragen's definition matches with what I think of when the term "S-expression" is used. Of course it's so minimal that the standard syntax of any non-toy lisp will have extensions.

FWIW the most minimal attempted standard of an s-expression would be rivest's basic s-expressions, which, as an extension of djb's netstrings can represent arbitrary binary values:

    <sexpr>     :: <string> | <list>
    <string>    :: <display>? <simple-string> ;
    <simple-string> :: <raw> ;
    <display>   :: "[" <simple-string> "]" ;
    <raw>       :: <decimal> ":" <bytes> ;
    <decimal>   :: <decimal-digit>+ ;
      -- decimal numbers should have no unnecessary leading zeros
    <bytes>  -- any string of bytes, of the indicated length
    <list>      :: "(" <sexp>* ")" ;
    <decimal-digit> :: "0" | ... | "9" ;