They have stated that they're more than willing to work with any manufacturer who is prepared to make a device that meets their security standards. But as it stands, Pixels are the only unlockable devices that meet that standard.

Someone like Fairphone could do pretty well from a tie-in with Graphene. But it's up to them.

Not that they locked themselves but other vendors locked and isolated themselves by not provideing enough hardware and software security measures so graphebne will be able to strenghten them

They haven't locked themselves to Google hardware by choice, it's the only hardware with the security features they need so far.

Remember graphenes goal is a very secure OS, not just a more private degoogled one, for that there's /e/os and other options that support more hardware.