Only if the function is defined, and only if there is serial data to process, and only if the end of loop() is actually reached. Calls to exit() make the last part the most unlikely event to actually happen..
You don't know that its not defined. The whole point in a function that is supposed to stop everything is that it stops everything and doesn't leave open the possibility that the user has left executable code that can still be run.
You'll also notice that in that example exit() is not called, but rather he has used a goto; statement to break out of a loop.