Not to forget that while GrapheneOS provides a default position of no privileged access for Google Services. This is not the focus or scope of the project as a whole. We simply give users the choice to use them or not properly sandboxed and isolated as any normal unrpivleged app. Therefore use of their hardware (which is the baseline standard) by the project is not antithetical to it.
Read https://grapheneos.org/faq#supported-devices
We can only support devices that properly provide the necessary standard of hardware, the regular firmware updates for it, and provide complete support for alternate OSes. (Flashing, locking the bootloader, verified boot, and able to take full advantage of the hardware.)
Discussion
my point was that you can restrict Google as much as you wish, at the software level, however if they are responsible for assembling the device, whats to say they don't simply backdoor you at the hardware level?
Pixels have the most external privacy and security research out of any of the available hardware choices. There is no evidence of any backdoor and it would not be logical for there to be one. There is no more secure or more trustworthy/verifiable hardware available regardless.
A sophisticated attacker doesn't need a backdoor. They can and do exploit vulnerabilities.
Google Project Zero recently decided to help secure Samsung cellular modem used by Pixels via offensive research. In a few months, they developed multiple remote code execution exploits.
Pixel devices are also arguably the most open in hardware based on the secure element and TEE.
However you're right that whatever device you use you're trusting the hardware and the OEM. However for every device you decide to put up against it, other than maybe the iPhone you could probably find more reason to be concerned over them, due to higher level software vulnerabilities and bad practices not having the privilege of just focusing so much energy and attention on hypothetical bsckdoors.