The parent's point is that possession of a physical good is a bright line separation. For digital files, there's a huge difference between [Files you own] on a hard disk, and files [on a hard disk you own]. There are files you can put on a hard drive that you don't own and will ultimately kill themselves when specified criteria are met, like DRM'd ebooks.
I would argue that the files on your hdd that can expire or made unusable by some remote third party are as incomplete as a book that is missing half the pages. For example a keepass file without the password/key-file is incomplete, the same goes for Audible aax files that can not be played without per-user 'activation bytes.' You have possession of the file but you never owned its contents.
For Audible I use OpenAudible which converts the aax files to m4b when I download them.