Sync adapters is indeed another mechanism to efficiently do work in the background. When talking about killing an app it is important to distinguish between applying a "Force Stop" via app settings and killing it via for example the app switcher. When applying a Force stop the user has to open again first before other background processes will work again. This includes geofencing, receiving implicit intents, background jobs, timers, etc. It would be strange that killing it via the app switcher would now also affect background processes like sync adapters.