Complicating the
matter is that a) I'm also seeing data that suggests that Smashwords only supports EPUB2, and b) right now, the tool I have available to me for generating EPUBs,
i.e., Calibre, doesn't talk EPUB3 either.
To make
matters all - the - more
complicated, Microsoft's Xbox Chief, Phil Spencer, has gone on record saying that new games aren't necessarily required to support the new hardware,
i.e. there's no mandate for developers to take advantage of the Xbox One X's capabilities.