In Deling City, while she was possessing Edea, Ultimecia tried to kill Rinoa. No-one in their right mind would try to kill their past self. Now, many people say she was simply suffering from the effects of GF amnesia. Yet these same people say she wanted to compress time to be with Squall forever. They also say that her "Remember your childhood..." speech proves she knows who they are.

So basically, the R=U theory depends on some really selective amnesia from Ultimecia. After all, Edea had full knowledge of Ultimecia's plans and motivations; no hint of R=U was ever given by Ultimecia.