> …Google could trivially make it work with GrapheneOS (which is more secure than any other Android OS on the market) but they won't.

But if Google did support third-party attestation, would the GrapheneOS Foundation be happy? Most of the thread seems to be a call for attestation to die, which feels impractical and unachievable. But "Google could use it to permit GrapheneOS for Play Integrity if that was actually about security" seems to be the real ask, and that seems reasonable and achievable. If that's true, I think it would’ve been more effective to lead with that and focus on it.

It is not only about Google. Its also about the App developers. Nothing prevents them to use the non-google attestation, however they decide not to use it (for many reasons). First time you actually notice this is when you installed GrapheneOS (attestation OK and bootloader locker) and some apps complain about a modified/rooted/... device. Another thing is, that you are warned by your Google device while booting that something is "not OK".