I don't think that's necessarily true. It's actually kind of confusing to me that Google doesn't whitelist GrapheneOS. According to their website [1]:
> GrapheneOS not only upholds the app security model but substantially reinforces it, so it cannot be justified with reasoning based on security, anti-fraud, etc.
Ever since I discovered this article, as a GrapheneOS user, I've felt sort of conflicted about it. On one hand, it would be good to see more apps support GrapheneOS. On the other hand, as a FOSS advocate, it irks me that I can't make arbitrary changes to my OS without taking the risk that certain apps might stop working for no reason [2].
It also places GrapheneOS in a bit of an awkward position as a open source project, where it's possible that they might be forced not to include certain features because they made this promise to banking apps. For example, they would probably not accept a location spoofing feature that can't be detected by app developers, because banking apps could no longer trust the OS-provided APIs.
But again on the other hand, this policy means it's not off the table that Google might whitelist GrapheneOS in Play Integrity one day, which I think would be a good thing (of course there's business interests at play here, but there's no valid non-business reason for them not to).
Overall, the entire mobile phone ecosystem depresses me. I just try to avoid using my phone as much as possible, and keep the number of installed apps to a minimum. I'll probably give the iPhone a try if they ever start allowing sideloading (wishful thinking). Unfortunately it seems like it may become unsustainable to avoid placing full trust in your phone's manufacturer, and I'd only be comfortable giving that to Apple, not Google or Samsung.
[1] https://grapheneos.org/articles/attestation-compatibility-gu...
[2] It should be noted that, as of today, I don't think any banking app has added explicit compatibility for GrapheneOS. AFAIK all the banking apps that work with GrapheneOS also work with any (non-rooted) open-source Android alternative. As a sidenote, I currently work around this issue entirely by simply using my bank's website instead of their silly app, and this will continue to work as long as hackers continue to fight against Google's dystopian plan for the web: https://en.wikipedia.org/wiki/Web_Environment_Integrity