The issue is more related to the intent of the action regardless of the practicalities of it. Essentially the subjecting giving a password is giving their consent to the device to unlock it. Using their iris or facial pattern as something compelled in that context would be distinct from the gunshot residue or blood of a victim which also incriminates them since that is something about them as opposed to being compelled to testify against themselves.
It is admittedly a weaker argument from a pure physical standpoint but the law doesn't operate based upon that anyway. Any judge would throw out a request in discovery for the defendant to produce a recording confessing their guilt even for legitimate comparative purposes and many other requests or demands that would be unduly prejudicial for 5th amendment reasons.
I'm just not seeing a meaningful distinction between forcibly taking their fingerprint ("something about them") to check against prints found on a gun and forcibly taking their fingerprint ("something about them") to check a smartphone found on the scene.
If they have a warrant to check the phone, and if taking fingerprints of the arrested without consent has already been found constitutional, I just don't see a problem.
I'm guessing this ruling is about taking the suspects actual finger and placing it on the scanner.
This is different from building a fake finger based on fingerprints and using it to unlock your device.
Consider the password case, you can't be compelled to give your password, but the government is allowed to throw all of their hacking prowess against your device to crack that password. They can even ask you about the name of your first pet to get through security questions. Though they can't compel you to answer truly regarding the name of your first pet.
It is admittedly a weaker argument from a pure physical standpoint but the law doesn't operate based upon that anyway. Any judge would throw out a request in discovery for the defendant to produce a recording confessing their guilt even for legitimate comparative purposes and many other requests or demands that would be unduly prejudicial for 5th amendment reasons.