HelloWorld
I just figured out that for an app I use I needed to turn off the "reroute the location requests" feature and turn on precise location.
You shouldn't ever need to do this. You might just need to bootstrap getting GNSS-based location via an app like GPSTest if the app doesn't wait long enough for it since it assumes network-based location is available. Our most recent release that's about to head into the Alpha channel has our own opt-in network location implementation.
now I'm under the impression that all apps needing and running through sandboxed Google play are able to permanently track my real location?
No, not at all. Location permission still works fine.
Or am I missing the meaning of this graphenos privacy feature?
Yes, you're missing the point of the feature. It reroutes requests from apps to the OS location service. This avoids needing to grant the Location permission to Play services by default despite many apps using the Google Play location API instead of the OS location API.