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.
Tracker blocking uses flawed heuristics. The only methods that are typically used are static lists which is just badness enumeration. There is nothing stopping the app/service from sending the data down a different domain that isn't blocked or a domain that can't be blocked without breaking the service.
Adding to that, how do we even decide what is a "tracker"? What is the definition? Some might say it includes all telemetry or crashlytics. Are those inherently malicious?
I don't think it would make sense for GrapheneOS to include something flawed like a "tracker blocker" that lulls people into a false sense of security. They use robust and meaningful methods for improving the privacy and security of the OS.