IIRC (and far beyond the details of the median news story) this case revolves around whether you can be compelled to reveal the code to access the files, as so doing demonstrates that you do in fact know the code and had access to the files. In other words, a 5th amendment issue. But that the specific facts of this situation were that the person had shown access to the files in front of CBP already, thus had already revealed he had the access.