I won't try to define this program for you here - you'd have to use it to believe it anyway. But basically, it is a LISP engine geared towards editing. You should check it out!
Thinking about the program as an example of free CBPP - well, it certainly is an example. But thinking about it as an example of CBPP of free content – is a little funky. The "contents" of Emacs primarily consist of its code.
Emacs is marketed as being "self-documenting" which means that included in this code is some documentation (not quite literate programming level documentation) that becomes accessible to the user from within the program when the program is compiled and run.
Furthermore, accessing (and, if you wish to, modifying) the source documents from within the program is relatively easy. Immediate access to its own source documents is something that sets Emacs apart from most software projects, free as well as non-free.
Whereas many programs provide online help, Emacs is one of the few that I can think of that actually provides a guide to its own source code. (Eventually, the HDM will include a comprehensive Guide to the HDM, and probably numerous increasingly complicated nested guides that help you "drill down" to bottom layers of mathematics.)
So, despite being kind of funky, I think that the perspective of the Emacs project being about CBBB of free content as valid, as long as you accept the idea that source code can be content.