Though I never bought into this theory at all I can understand how it developed. FF8 was a game where virtually only two characters received much development. Everyone in the game outside of Squall and Rinoa needed a lot of personal conjecture on the players side in order to fill out their personalities.

Tell me what do we really know about Quistis?

What do we really know about Zell?

What do we really know about Irvine?

What do we really know about Selphie?

THe sorcerers in the game were even worse than those characters as they had virtually no development whatsoever. All the sorcerers really were were plot devices put in the game in order to reach a particular goal at a particular stage. When characters receive such little development that creates situations ripe for a lot of theorizing and since in Ultimecia's case basically the only appearance she had was the final boss battle people are naturally going to try to give that underdeveloped final boss some development. And virtually anything could fit into theories that people created because Ultimecia was a completely blank slate from a character standpoint.