Maybe I missed it, but assuming GrapheneOS doesn't adhere to this verification, or provides some OS-level way to disable it, what makes Graphene worse after this change?
Apps that require Google Play Service or some form of attestation will not run on a Linux phone either.
GrapheneOS only runs on the Google Pixels, and Google may decide to render future Pixels unusable for GrapheneOS (e.g. by preventing to unlock/relock the bootloader).
But another Android manufacturer could get to the point where GrapheneOS endorses them. It feels like it shouldn't be that hard for an Android manufacturer, and they would immediately get quite some attention. Maybe not mainstream attention, but largely profitable, I think.