Today I was checking Nobuo Uematsu’s fansite and discovered that a Final Fantasy XIII theme was added to the sheet music. I printed it and it appears to be the main menu theme (also in the demo) “composed by Nobuo Uematsu/Arranged by Arnold Morrison.” Whether this is a mistake or whether it is indeed composed by Nobuo Uematsu is currently unknown, but in the past all of Uematsu’s sheet music has been arranged by Arnold Morrison.
Uematsu-sama originaly would compose the main theme for Final Fantasy XIII but because he was too busy with “other works” (=FFXIV) he passed the main theme to Masashi Hamauzu. According to Square Enix, Hamauzu composed the entire soundtrack of Final Fantasy XIII.
Rumor, mistake or truth, at least we have the sheet music available and arranged perfectly by Arnold Morrison (as usual)
You can download it on the sheet music page