>    João "who instantly regrets mentioning the manual cause I just say
   > unibyte-string there, too"

   Yup: `multybyte-string-p` is not even mentioned in the manual.

That is true.  But that isn't the same as multibyte-string-p. :-)

(elisp) Text Representations:

 -- Function: multibyte-string-p string
     Return ‘t’ if STRING is a multibyte string, ‘nil’ otherwise.  This
     function also returns ‘nil’ if STRING is some object other than a
     string.