Pure wild speculation if I'm honest, however I'd be more surprised if I was completely wrong. It's always seemed sketchy the way Google have basically said "Use our phone, it's more secure!" with their Nexus and Pixel phones - this was long after the time Google stopped not being evil. At best, the security problems have simply changed manufacturer. Also, Google have a history of undermining development of circumvention, eg hiring the developer of MicroG and forcing him to stop development as a term in his contract.

The diva part is widely known, GrapheneOS developers don't play nice with the rest of the custom development community. So, while I can't substantiate any actual deal between them and Google, it's the perfect recipe.