I just listened to a couple of free previews. I don't know how many voices they've got but the female one you could definitely tell was AI, it had the weird stilted voice where the timing is off like Siri. The male voice I'm listening to at the minute is a lot better and I'm not sure you'd be able to tell if you didn't know. Occasionally you get a little bit of that 'robotic digital' sound but it's pretty good. I'm impressed at how it can understand the text and take an appropriate emotional tone such as when a character is experiencing doubt and asking a question.
Female Voice: https://books.apple.com/gb/audiobook/shelter-from-the-storm/...
Male Voice: https://books.apple.com/gb/audiobook/pale-moon-rising/id1640...