If you say only Lisp can be used to implement Emacs, would you mean a specific Emacs or an editor belonging to the larger family of Emacs-like editors? When I would speak about how to implement "an Emacs", I would include the option to use different programming languages. For example I could imagine that C and Python is a valid combination, even Python alone would be an option. Python can be used interactively, which would be sufficient for an interactive extension language.

Here is an old Emacs timeline:

https://www.jwz.org/doc/emacs-timeline.html

The actual historic Emacs wasn't written in Lisp and was not extensible in Lisp. It was written on top of TECO and assembly. It was extensible.

The second and third Emacs were both written in Lisp (Zetalisp and Maclisp) and they were completely written in Lisp.

At some point Gosling wrote an Emacs in C and Mocklisp. Stallman took that one and rewrote it into GNU Emacs.

We have lots of Emacs variants written in languages like TECO, C, Fortran, ...

Craig A. Finseth wrote "The Craft of Text Editing: Emacs For The Modern World"

http://www.finseth.com/craft/

Chapter Ten of above book describes what Emacs-type means: http://www.finseth.com/craft/#c10

Extensibility is a general feature and not tied to Lisp or GNU Emacs.

The book contained a list of Emacs implementations. An newer list is here: http://www.finseth.com/emacs.html

You can see that there is a multitude of editors in the Emacs category. The list also mentions the implementation and the extension language.

> GNU Emacs is what people usually mean when they speak about Emacs

That's a bit sad. It's like saying "Linux" and think that its the same as "Debian Linux". Similar there are a lot of different Emacs-like editors. Claiming that there is only a single way to implement Emacs goes against the evidence that there are a lot of Emacs-like editors, which are not implemented in C + Emacs Lisp, including the original first Emacs.

I would think that by far the most important Emacs is GNU Emacs, but I don't think its implementation language choice (C + Lisp) is necessary to implement an extensible Emacs-like editor. Also be aware even though GNU Emacs is a popular Emacs editor, there are some people who are using different Emacs-like editors instead. I typically use a Hemlock variant written in Common Lisp and Zmacs, written in ZetaLisp. Both core designs date back many decades, actually even before GNU Emacs existed.