Because what Donald says is law. Just because he said that doesn;t mean anything. Besides, do you really think he would say "Oh you were alright for a beginner!" to Sora's would be love interest that isn't Riku?
Also, all she did was smack like two or three Heartless and that's all. If she did anything off-screen and they try using that, it would just be plain retarded. Plus, I don't see what that would really have to do with anything to be honest.
Lon, why would they go to all this trouble of retconning to just say she has potential and then go but we're not going to make her a weilder?