Well it’s an option you need to turn on, so I assume those users without fingerprints set up don’t (can’t?) turn the option on and just carry on being let straight in to the app.
Well, there’s the option in the corner to log out, and then I assume you can log back in with email. So not as convenient as a PIN if you’re e.g. in the middle of cooking and have greasy/doughy hands, but not exactly a lock-out.