He may in some cases be forced to surrender a key to a strongbox containing incriminating documents, but I do not believe he can be compelled to reveal the combination to his wall safe —- by word or deed
Meaning the fingerprint gathering for the use of opening the phone is tantamount to compelled testimony in the general case, while the fingerprint gathering for the use of identification and matching is not.
Keys don't change. Fingerprints don't change. A biometric identifier is therefore not affirmative.
Combinations can change. Pin codes can change. Utilizing either requires active participation in a process. And is therefore affirmative.
Fingerprint usage is therefore tantamount to using a key, and if you are stupid enough to use a biometric identifier as an access method, you've picked a non-affirmative access method.