C-x ESC ESC (translated from C-x <escape> <escape>) runs the command repeat-complex-command

Old timers (like me) use it for meta. Ask anyone over 50!

As the meta key, i.e., hitting ESC then x for M-x.

On a Mac I use "option" for this; much shorter reach

It's funny but I've been using Emacs for almost 25 years and still use ESC instead of ALT/OPTION, guess that's muscle memory from using Emacs via telnet sessions.

Alt+Key doesn't save any keystrokes over Esc+Key anyway, and I think the Esc version is easier to type.

Especially things like Esc : (which is Esc then Shift-;) is surely easier to type than Alt-Shift-;.

That's my experience, too, I guess it's from learning EMACS and its ancestors on actual VT-100 style terminals with no meta keys.

Indeed, never even crossed my mind to set the escape key to meta, definitely not a default even for Cocoa emacs.