I read it the exact opposite way: you are forbidden from using facial and ID scans solely for age verification (as the OS-provided signal shall be the primary indicator of age), but if you already need to obtain the user's age for other reasons using more reliable means (say, a banking KYC law requiring ID scans) you are not required to discard this more reliable source in favor of the OS-provided signal.

I read it as you're not allowed to discard the more reliable source.