The interpreter we wrote in that actually wasn't written for Emacs, it was written for TECO. It was our text editor, and was an extremely ugly programming language, as ugly as could possibly be.
TECO was so ugly because the command language and the programming language were the same thing: You programmed TECO by entering the same commands you used to do editing with it, and those commands were both terse and made heavy use of control characters.
On the other hand, it was also a high-level programming language with native support for strings and regular expressions and, of course, the editor qualified as an interactive interpreter. TECO-the-language actually got reasonably popular within the communities that used it as an editor, especially the ones that had the more advanced TECO variants with all the bells and whistles.
For proof, see the source code here: http://pdp-10.trailing-edge.com/mit_emacs_170_teco_1220/inde...
It is certainly heavily annotated for good reason.