GrapheneOS is not a "Google ROM". GrapheneOS is based on the Android Open Source Project, just like this project, just like LineageOS, and just like nearly every OEM OS. GrapheneOS is not affiliated with google and is not based on the google pixel stock OS.

pretty much every ROM is based on AOSP, that doesn't say anything new, the point was both Google and Graphene ROMs are barebone lacking basic features and have zero customization