I was a bit confused when you mentioned this, but then I realised the actual sequence is ^X^C. It's because ctrl-X is a sort of metacharacter, and ctrl-C is used for the standard INT signal you know from other programs.
And to be fair, GTK emacs has File->Quit as well (which helpfully reminds you you could use C-x C-c instead!).
And to be fair, GTK emacs has File->Quit as well (which helpfully reminds you you could use C-x C-c instead!).