If the government already knows about the evidence, they don't need me to provide it.
The issue arises when the officials know about the evidence but they don't have direct access. For example, you keep a set of books for illegal activities. An undercover agent saw the books when you were interacting with someone but didn't get a look at the whole book. That's an example where they can compel you to produce the books even though they don't know what's in the books. Or, an officer pursues you into your house, and sees you throw something in a wall safe and lock it. You can be compelled to open the safe in that case. But, for example, if that same thing happens but you went into someone else's house, tossed it in their safe and locked it, you could make a reasonable argument that you can no longer assist in discovery because you don't know the combination to a safe you don't own, and in that case you can't be compelled under contempt to provide the combination to that safe.
In the case at hand, the prosecutors couldn't prove that he actually had access to the decryption key for the device and having that key would implicate him, so they can't hold him in contempt if he says he's unable to provide it.