Using a back button will not force stopping an app is. Most versions of Android will let you force stop an app by going into Settings, choosing a section about apps, choosing iNaturalist, and tapping a “Force stop” button. On my Pixel 3 running Android 12, I can also do this by opening the app switcher and swiping up on the app I want to quit. The reason we ask you to do this is that there are certain tasks, like file cleanup, that only happen when the app is started, not when it is re-activated after being in the background.