TTS tools exist on all computing platforms these days. The difference with this recent release is just that it sounds better. Apple would not be creating a derivative work, whoever uses it would be.

EDIT: It has occurred to me that they may have signed a deal as part of Apple Books that says they won't release tools of this nature. I don't know if that is the case, but that isn't the scenario I'm talking about. Just the case where there is a tool that can do high quality TTS. I do not believe that violates any copyright since it's a tool.