I guess there's actually nothing stopping you from rooting: you say "nope" when they ask you to confirm re-locking the bootloader, and then do the usual shenanigans with patching and flashing boot partition.

However, it makes graphene a whole lot less grapheny since you can't re-lock the bootloader anymore (except if you sign modified stuff yourself and let vb know of your key, which sounds like too much of a hustle), which means you don't really need a pixel and graphene except for a few unique features mb.