> Don't forget this is a text editor, not an IDE or some general purpose automation harness

Technically, it is just a text editor. It was created to be a text editor, so you are right, but up to a point.

Emacs is written in lisp, what it means? It means that the only part of emacs written in C is a lisp machine. And even not the whole lisp machine is written in C: there is a rudimentary read-eval-print loop (REPL), and maybe a bytecode interpreted. This REPL I believe is just to boot up purposes and is replaced with proper lisp implementation on later boot up stages. It is not lisp added to a text editor, it is a text editor added to lisp. It means, in particular, you can add to that lisp not just text editor but anything else too, like mspaint for example.

And people do just that. They write software in emacs. In particular they wrote IDE in emacs, so while emacs by itself is not an IDE, you can turn it into one.

> some general purpose automation harness

Oh, it is. It is more general than any other automation harness you know. It is a lisp machine, you can automate anything in it. Well, technically you can, in practice it can take take too much time to be practical. In practice when choosing automation harness, you'd prefer not the most general, but the most specific harness for your task.