I think it's more of a marketing claim from less secure systems that "privacy is not security, and GrapheneOS focuses on security while we focus on privacy".
GrapheneOS does care about both, quite obviously. And GrapheneOS tends to say that if your security is bad, then it is affecting your privacy too. Whereas others say "sure, we break the Android security model by unlocking the bootloader and signing our system with the Google test keys, but your apps will contact Google through microG instead of the Play Services, so it's more private". Which is worth what it is worth...
This is only my opinion, but GrapheneOS's approach to privacy seems obtuse to me. They will claim that an unlocked bootloader is a risk, but then turn around and recommend you install proprietary apps GApps in their sandbox. The sandbox doesn't matter if all the private data is in the same sandbox!
Reminds me of https://xkcd.com/1200/
> I think it's more of a marketing claim from less secure systems that "privacy is not security
I'm not sure Cyanogenmod had a marketing team that convinced me of anything when I first installed their rom in 2013 and explored my phone's capabilities with root. Accessing the sensor devices, inspecting what the different apps do, what the OS is doing, installing Xprivacy to provide fake data to tracking apps... none of that is possible on GrapheneOS, you can only use the Android APIs, same as on stock
Am I brainwashed by marketing?!