There's nothing GrapheneOS-specific about it and it doesn't prevent rooting. LineageOS doesn't officially support it any more than GrapheneOS does. It doesn't stop people doing it for either. Our recommendations aren't law.

Any files created by apps in their main data directories are inaccessible on most distributions of Android (I think it is actually required to be Google certified). The exception is apps that go out of their way to store files in user accessible directories or provide a feature to export or share data out of the app.

By rooting your device you can access the app data directories as you wish.