Just because the PS4 can run Linux doesn't mean you get free access to data on arbitrary discs. If this is anything like the PS2 and PS3 incarnations you're locked out from a lot of that.

This isn't an officially sanctioned sandboxed Linux mode like on PS2-3. It's an exploit for (an old version of) the customized FreeBSD kernel that all games on the PS4 run under; in this case the exploit is used to throw away the kernel in memory and chainload Linux, but it could also be used to just install some kind of hook that dumps decrypted game data.