They still use a subscription model it's just a non-recurring one.
https://help.audible.com/s/article/do-i-keep-my-credits-if-i...
Do I keep my credits if I cancel my Audible Premium Plus membership? No. If you end your Audible Premium Plus membership, your credits will be lost with your other membership benefits.
This and other privacy-related issues (see my comment history) is why I won’t consciously use Amazon again.
Dark patterns all over.