It doesn't really.
ZMacs and the Listener on the Lisp Machine is just as single threaded as GNU Emacs. One ZMacs frame cannot run the above code without locking up, one Listener instance cannot run the above code without locking up, if you open a file in ZMacs it will lock up while reading up the file.
The only difference is that when the ZMacs background process is working, you can still talk to the Lisp Machine in other ways.
The closest you might get with GNU Emacs is running multiple versions, one for each "program" (Emacs, mail, web browser, ...) -- still not exactly the same thing since you can't easily poke into the mail program from another instance.
At the end of the day .. none of this matters much, as we can evidently see since nobody has really tried to fix this.