Slippery slope being a thing, a ruling here has the potential to affect YouTube as well, since they run automated captions on videos. I’d be surprised if Google didn’t step in an offer up an opinion in support of Amazon.
The difference between an audiobook and a video is pretty small in a significant portion of YouTube content.
As the GP said, there's quite a few youtube videos with a completely static background where the content is the speaker's words.
AFAIK, they would have to obtain prior permission from the copyright owner before re-distributing a modified version of the copyrighted work.