GrapheneOS is an Android/Linux distro, not GNU/Linux or musl+busybox/Linux; I suspect most of their security work isn't portable to the unixy Linux distros.
I read this whole thread again and it seems that the answer to the original question, is: the pixel drivers are maintained outside of the kernel tree by Google, and not these Graphene folks.
Sounds like I should complain to them instead. Yet they are known for being unreachable.