Verified boot does indeed make this more complicated, but it's totally possible to build Graphene with your own signing key and get full control over the OS that way (i.e. https://github.com/schnatterer/rooted-graphene).

Looking at their public statements on the matter, it seems like the problem isn't exactly that they treat the user as a potentially hostile actor so much as that they treat the system UI and persistent storage as a potentially hostile actor (though I admit from a practical perspective that's nearly the same thing): https://www.reddit.com/r/GrapheneOS/comments/13264di/is_root...

I wonder how they'd feel about something like protected confirmation to enable sudo: https://source.android.com/docs/security/features/protected-...