Didn’t emacs get developed on ITS or Multics? Seems Unix just imported it, but your point still holds.
Early versions of Emacs didn't have lisp extension languages, which is kind of strange to think about considering the role elisp plays in GNU Emacs as we know it today. GNU Emacs with elisp is something that developed in the context of the MIT lisp machine scene. Sure it runs and was developed on unix-like operating systems, but lisp extensible emacs were never really some sort of embodiment of the 'unix principles'.
The first four Emacs editors, which had something like an extension language: Emacs (1976), EINE (1977), SINE and Multics Emacs (1978). EINE and Multics Emacs were written & extended in Lisp. SINE was using a 'lisp-like' language. A bunch of further Emacs-like editors appeared then.
GNU Emacs later (1984) was based on Gosling Emacs (1981), which was written in C and extended with Mocklisp. Mocklisp was a rather uncomplete Lisp, which was replaced with Emacs Lisp by Richard Stallman.
https://en.wikipedia.org/wiki/Emacs#History
Yes.
Emacs development began during the 1970s at the MIT AI Lab, whose PDP-6 and PDP-10 computers used the Incompatible Timesharing System (ITS) operating system that featured a default line editor known as Tape Editor and Corrector (TECO)
The UNIX philosophy isn't meant to be taken as gospel - hell, it's very hard to define what "doing one thing" even means.