It is a classic joke. Emacs is often a little behind competing text editors (Vim has better keybindings IMO, things like Visual studio have better IDE support, Atom text editor turns out to have better syntax highlighting, etc, etc).

So, the line of the joke goes, it must not really be a text editor because it is not very good at it and has a wild array of other capabilities. Like "M-x dunnet". Must be an OS.

Of course the reality is that Emacs just copies good ideas from other people. A pretty normal Emacs setup uses Vim keybindings, LSP and now tree-sitter to get a good experience programming.