No, it's similar to attestation APIs like android SafetyNet (now called Play Integrity API) that are used to check that "your ROM is valid according to Google".
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.
A trust chain beginning at the bootloader is what will ultimately enable this API, though, because that's what SafetyNet/Play Integrity API relies on. If you don't have a locked bootloader, or you're not running stock Android, you won't pass SafetyNet/Play Integrity (at least the higher tiers of it).
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.