This is a misleading way to put it.
Re: Android.
Goggle can supports AOSP attestation like any other vendor who wants to support it. They invented it.
So instead of immediately locking down everyone using android to ONLY Google-dependent method, I'd developers could go the vendor agnostic way, but consciously decided not to.
It's untrue to claim that supporting AOSP attestation only serves GrapheneOS and leaves out everyone using Google-surveiled handset.
Nb, mixing it up with Apple is a conscious way to further the false claim, and I believe it's not accidental since these ecosystems are naturally completely separate.