M-expressions aren't limited to explaining evalquote in this manual
I know. The m-expressions are supposed to be Lisp. The s-expressions are supposed to be an implementation detail.
The glossary eludes to part of why m-expressions are dropped.
Lisp was supposed to be m-expressions compiled to s-expressions. Humans were never supposed to be dealing with s-expressions directly. M-expressions weren't really dropped. They were still included in the manual several years after no one finished the language.