You don't even need the Ultimania guide to prove that Rinoa is not Ultimecia.

Ok, if it were true, Rinoa likely wouldn't know if she was Ultimecia, but Ultimecia would definately know she is Rinoa. Ultimecia tries to kill Rinoa at the end of disc 1. If Ultimecia is Rinoa, killing herself in the past would negate her existence in the future and her whole plan for time compression and consuming all of the pieces of the Great Hyne would be shot. If Ultimecia were Rinoa, she wouldn't try to kill herself. End of story.