Years ago it was remarkable for software to have docs built-in as Emacs does.
Then for many years it was standard for software to have help files, and it seemed anachronistic for Emacs to loudly proclaim it is self-documenting.
Now in the Web and LLM age, much software doesn’t even try to have built-in help or even much documentation, and it’s again remarkable that Emacs is self-documenting, especially the part of Emacs that users can program.
Emacs' notion of self documentation refers to something slightly different than the fact it has online help files. The help facilities can query the Lisp runtime for things like functions and keybindings. These update dynamically as the system is reconfigured. The result is something that isn't quite as cleanly presented as an online help document, but has the benefit of being deeply integrated into how the system is actually configured to behave at the moment. Very cool, and very much dependent on the open source nature of emacs.