There are no "Emacs experts". Bedrock of Emacs is Lisp. Lisp is the essence of computation itself. It's both simple to understand (5 basic special forms) and impossible to master at the same time - you can construct entire universes with those 5 basic building blocks - quote, if, lambda, let, and set. If someone finds something cannot be achieved in Emacs they either are wrong, or wrong at the point in time - theoretically, anything can be done in Emacs, it's just a matter of time. So, technically, it's impossible to capture all possible features of Emacs, the totality is infinite.

In comparison most other languages are 'closed' - e.g., C is a closed language. Its spec is finite and fixed (C99, C11, C17, etc.). You can genuinely master it: all keywords, all standard library functions, all undefined behaviors, all edge cases. There's a ceiling.

Lisp is unusual, The language itself is a tool for language-building. Lisp is 'open'. There's no canonical "complete" set of what exists. Thus there's never completion or "mastery"