Google, by giving the Pixel 4 face unlock but no fingerprint sensor, has effectively determined that the two methods are biometrically equivalent. That being the case, they should have simply fixed the API so that any call for biometric authentication would use whichever hardware is available on the specific device the app is running on.
Doing it that way would force developers to opt out of biometric authentication entirely, if that's what they wanted to do.
Optionally, the API could add a parameter to specify biometric fingerprint authentication (let's call it option 1), faceunlock (option 2), or "any" (option 0) with option 0 being the default mode. Existing apps would default to "any" (option 0) and developers would be more inclined to get off their collective butts and update their apps if they wanted something different.