GrapheneOS (like any other AOSP fork) is technically a Linux based OS. They run a modified version of the Linux Kernel. What matters is the changes they have made to the kernel, as well as enforcing AVB, SELinux, etc. etc.
“Linux” phones that run modified desktop Linux distros are hugely insecure devices that lack many basic security and hardening features.
Open source or source availability is not a requirement for auditing a system. There would be evidence that would have almost certainly been found by now if this was the case. It is up to you, or the claimant, to prove their claims. I can say that there has not been any evidence of data collection by hardware components found, despite years of Pixel devices being tested by security researchers and mobile forensics companies. Not only that, the actual technical capabilities of the hardware (isolated component without networking capabilities) backs that up.
What do you have except fearmongering?