That’s my understanding as wellI didn't think they could? Biometric authentication is delivered by the phone by whatever method is available to it - FaceID or TouchID, it doesn't distinguish and neither does the app. So either biometric authentication is on or off?
At least that's what I've read and heard. I know there was a lot of misinformation about this posted on places like this when the phone was announced, but Apple have been pretty consistent about it from the start I thought?