

States the information is in cleartext but then explains how everything is encrypted (in transit).
That’s not how I understood it. The message context is allways encrypted in transit (using a novel encryption scheme). The auth_key_id however is not encrypted. And that can be used to track users as it is s(semi-)static.
Similarly (for Germany & Austria at least) there is also Thalia who offer DRM-free mp3-downloads of audiobooks - for purchase or through a subscription similar to audible.
(Note that this is not your next-door bookstore and quite a big chain not without controversies)