Something like SLIME isn't essential to execute Lisp code, you're right, but it is essential in order to understand the efficiencies afforded by interactive development, which may as well be one of the pillars of the language, since "garbage collection" and "an object system" aren't in and of themselves differentiators anymore*. Using something like SLIME also takes a lot of the pains that people have with Lisp away, namely balancing parens and indenting code correctly. People who do the "I'll use my own editor" approach to beginning Lisp usually write things that look like:
(defun myfun(x)
(let (x)
(setq x 5)
(when (eq x 6)
(print "6")
)
)
)
____
* Of course, technically, CLOS is something to behold. But you won't sell someone on Lisp because it can do "OOP".
Interactive development is because Common Lisp runntime has the concept of images and systems. Just like you start you OS and do things with it, you start the Lisp runtime and do things with it. It’s a point A to point B type of interaction like running a script. Smalltalk, SMl, a modern web browser,… has the same kind of interaction. It’s not dependent on any editor.
Also automatic idiomatic formatting of lisp code is possible in any editor as long as they have the settings for it. Not sure how Emacs is the gatekeeper on that one.