There's a lot of speculation on this thread that is misplaced. Unfortunately even though my account is 12 years old I haven't posted much so I guess moderators have to approve every post.
You can see here that it is literally a bog standard 10th gen Kindle.
It's not running Android, just the normal Kindle OS. It is also actually running text to speech not some sort of mp3 file.
Anyhow, some things that I've discovered while my posts were waiting for moderation: I did a lot of experimenting with different files and formats and messing with different metadata.
What I found is that I could use Calibre to take the ASIN from a book that has working TTS and transfer that to any book (as long as the book's language is set to Chinese) and convert to the KFX format and the TTS would work.
It didn't matter if there are multiple books using that ASIN. But as soon as you alter that ASIN the Kindle no longer allows TTS on that book.
My theory is that the Kindle is storing a database of ASINs for books you have purchased (and allow TTS, Harry Potter didn't when I got it on Kindle Unlimited) and using that to determine if TTS is allowed.
I could be mistaken about how they are doing it, but that seems to fit with what I've found.
