Imagine you were to take the example further, unlocking the device required a sequence of fingerprint reads, with a precise ordering. i.e. left-ring finger, rig

Imagine you were to take the example further, unlocking the device required a sequence of fingerprint reads, with a precise ordering. i.e. left-ring finger, right index finger, right little finger, etc... That sequence would be a passcode, just as a precise sequence of keypresses would be. The government can insist on all your fingerprints, but not (in this argument) the correct sequence of uses of those fingerprints to unlock it. If it's only a single finger this same argument could apply.