It's not that you have a misunderstanding of the 5th amendment, it's just you were misled by the article summary and don't have the legal knowledge to know that it's wrong. In fact, the 5th amendment is not an issue in this case. It's more of a 4th amendment issue. The argument that worked in the last court is that the 5th amendment applies because the password is testimonial. The reason the defense worked is because they're right that it is testimonial in nature and is protected under the 5th amendment. Where the lower court messed up was thinking the 5th amendment was an issue.
Watch out, here comes an analogy. If police were conducting a search warrant on a home and there was a physical safe that contained actual photos of child pornography AND the safe is within the scope of the warrant. They could ask for the safe combination from the owner, but that's protected under the 5th amendment. It's protected because it's testimonial in nature. If he knows the combination of the safe then he is demonstrating that he owns it and likely knows what's in it. But let's say the owner waives his rights and opens the safe and lets the cops search it and they see child porn in it. Then, a police officer bumps the safe and closes it and they don't know the combo. Well now they have probable cause that you are in control of the documents in the safe and the 5th amendment is not the relevant protection.
The only possible interpretation of any research whatever in the `social sciences' is: some do, some don't. -- Ernest Rutherford