Secure boot can protect you eg. against malware gaining write access and modifying your system. I see it as user protection, as long as you can sign the trust chain. This is what GrapheneOS is doing as far as I know.
To take your GrapheneOS example, apps wishing to support it must add GrapheneOS keys: https://grapheneos.org/articles/attestation-compatibility-gu...
If this proposal goes ahead, it's unlikely that you'll be able to convince site owners and/or ad networks to add the keys of your open source OS.