'IBuriedTheBodiesAt123MainStreet' is self incriminating by itself. But not of the crime for which I am charged with and for which a warrant has been issued. It's a crime that the police may not be aware of. So forcing me to reveal it would be self incrimination and beyond the scope of the current warrant or investigation. You are not getting my pass phrase.
I think a defense along these lines was actually tried once. Can't unlock my phone. Because I might be guilty of other crimes, evidence of which might be on that phone. No, I'm not telling you what those crimes are.