I say it's not an audio book, but a book produced in computerized voice. Difference being that what I want in an audio book is much more than current technology offers. I want the level of human performance that no TTS system can provide now. Having listened to great many audio books, I know what I like. Books read by professionals like George Guidall, who is awesome. I have returned many books after 10 minutes of listening because they were read by some hack who went to nail-on-chalkboard school of book reading. Looking at you, Ron Silver. I would not even entertain listening to a whole book via TTS, it's a waste of time.
So, when TTS systems reach a point at which they can pass a touring test with me, then it's a performance and maybe writers guild has a point. Until then, the audio book market and their revenues from it are safe, IMO.