This approach works too. The following code produces exactly the same result as what I posted earlier:

(defun ox-texinfo+--untabify (string back-end _)

  (if (eq back-end 'texinfo)

      (replace-regexp-in-string

       "^\t+"

       (lambda (match)

         (make-string (* (length match) 8) ?\s))

       string)

    string))

(add-to-list 'org-export-filter-final-output-functions

             'ox-texinfo+--untabify)