I'd describe that as "verbing" it rather than "punning" it. The verb makes sense too, when you take keyboard short cuts into account. A very common short cut to exit a program (in Windows at least) is File->Exit, which translates to `Alt+F, X` so you do, in fact, "X it".
If ex is not your favorite editor, use EX<ESC><ESC> (at least, that is what I think Wikipedia claims. I'm sure any decent TECO user never exits their editor, browsing the web in a browser macro)
[I also checked ed and edlin. They had two commands for exiting: w writes and exits, q quits]