It was Motorola Mobility that reached out to the GrapheneOS project to propose a collaboration, previously, GrapheneOS’s own proposals for collaboration had not come to fruition, given this, Motorola Mobility’s proposal was a golden opportunity, and GrapheneOS was certainly going to accept it.
Fairphone will never be compatible with GrapheneOS. Fairphone has chosen to support and collaborate with for-profit companies that attack the GrapheneOS project (Iodé and Murena). Fairphone has made its own choice and taken a path completely opposite to that of GrapheneOS; they are also opposed to improving the security of their devices.