androidin
I don't think 2-factor fingerprint unlock is meant as a replacement to face unlock, but to allow a good combination of strong password and the more convenient combination of fingerprint+pin encouraging more people to use a strong passphrase and thereby increase security:
Our 2-factor fingerprint unlock feature will allow people to combine a strong passphrase with convenient fingerprint+PIN unlock to get the best of both worlds. We expect that to massively increase the proportion of our users using a strong passphrase, especially when we add our planned UI for generating random passphrases automatically where it can give you a few choices and you pick the one you like.
https://discuss.grapheneos.org/d/12848-claims-made-by-forensics-companies-their-capabilities-and-how-grapheneos-fares/6
Since the developers of GrapheneOS seem to focus their limited resources more on security than on convenience features, face unlock seems to have lost out (for now):
It's not currently planned particularly since we're going to be focusing on recommending the upcoming 2-factor fingerprint unlock feature and the UI for face unlock would be harder.
https://github.com/GrapheneOS/os-issue-tracker/issues/3549