Conspiracy theory time: GrapheneOS is a skunkworks project from Google, to sell more Pixel hardware.

Considering last years development and quite open Google hostility?

No.

GoS have provided a lot of patches upstream, Some of which were even applied. Despite that they wouldn't get early access to A16 just because. Access EVERY vendor promising to preinstall privileged Google services has.

Allegedly Google security team was very happy about that idea, but got vetoed by management.