It does mean that. You can't be forced to divulge information in your head, as that would be testimonial. But if there are papers, records, or other evidentiary materials that are e.g. locked in a safe you can be compelled to open it with a warrant, and refusal would be contempt.
I know it seems like an incredibly dubious claim but the "I forgot" defense actually works here.
It's not really that useful for a safe since they aren't _that_ difficult to open and, if you haven't committed a crime, it's probably better to open your safe for them than have them destroy it so you need a new one. For a mathematically impossible to break cipher though, very useful.
They are considered to be more like keys to a safe than private knowledge. They also can't be changed if compromised. A sufficiently unguessable PIN or passphrase is better than biometrics.
