That is the big chip in this rumor. The NA blog post didn't go into much detail about it but the EU PS Blog (from Bateman) said this:
"Final Fantasy fans will be excited to know that theyre not just getting a mind-blowingly beautifully HD version of the game they love, oh no! Both games are based on the International version, which is the most complete, feature-heavy iteration of each game to release. The International version of FINAL FANTASY X-2 never made it to European shores before, so theres quite a chunk of additional content for you to dive into!
The International version of FINAL FANTASY X introduces an Expert version of the Sphere Grid in which all the characters start in the middle and you can choose any path
but there are fewer nodes and therefore a decrease in the number of upgrades available challenging indeed! This edition also includes new abilities, new superbosses (Dark Aeons and Penance), and more.
FINAL FANTASY X-2 is also getting the International treatment, high definition graphics and more content than I can list! But youll have to wait a little bit longer until we reveal the full list of enhancements and extra features."
This slight ambiguity on X-2's content makes Final Mission a bit of a question mark, but this statement would be in very poor taste if it wasn't included on the disc at retail.