While it’s true that they will open a physical safe themselves if you refuse, you can indeed be held in contempt if you have the ability to open a safe and refuse to do so when presented with a valid warrant. The “physical safe” analogy is one of the things that’s (unfortunately) applied as an existing-law analogy to crypto.
That's actually only true if they already know for certain it's your safe and you have access to it. Otherwise, admitting that you know how to open the safe (by opening it or providing the combination) is admitting that the contents of it are in fact yours. That's self-incrimination and you can't be forced to do it, though of course with a valid warrant they can still try to break into the safe. They just can't make you admit it's yours, and that's what you're doing if you open it.
In this case, however, the idiot went and bragged to the police that yeah, that stuff is all mine! To extend the safe analogy, that's like saying to the police "Yeah, I know the combination, but I'm not giving it to you!" Now you wouldn't be telling them anything they don't know, so opening the safe is no longer self-incriminating. If he'd kept his mouth shut (first rule of being questioned by the police, keep your fucking mouth shut, they mean it when they say anything you say will be used against you), this case would likely have been decided differently.