You're fine to keep using Audible, in that case. This Twitter thread is overblown. Read through the top few comments here and you'll see that what is happening is exactly what the artists sign up for: they get a portion of the net sales of credits, weighted by length/price of the book.
Audible's business model is selling subscriptions. Artists get a share of that revenue, and their share is comparable to that from any other publisher. The Twitter user is trying to exploit internet outrage for their own gain, not reacting against a truly bad deal.
Would it be comparable to Spotify Premium crossed with a Jukebox service of some sort in that regard, or is the credits model unusually unique to Audible here?
Audible's business model is selling subscriptions. Artists get a share of that revenue, and their share is comparable to that from any other publisher. The Twitter user is trying to exploit internet outrage for their own gain, not reacting against a truly bad deal.