Calling QCoreApplication.quit() is the same as calling QCoreApplication.exit(0). To quote from the qt docs:
After this function has been called, the application leaves the main
event loop and returns from the call to exec(). The exec() function
returns returnCode. If the event loop is not running, this function
does nothing. [emphasis added]
So quit() or exit() are nothing like sys.exit(). The latter will terminate the program, but the former will merely terminate the event-loop (if it’s running).
When the user cancels the login dialog, your example should just call sys.exit() to terminate the program. Otherwise, your program will just get stuck in the blocking while-loop.