That's the first I've heard of that. Kind of reminds me of another theory that I've been reading about a couple of years ago, something along the lines of R = U (Rinoa = Ultimecia). Although it sounds plausible and could explain why Ultimecia's GF resembled the griever on Squall's necklace, there simply wasn't enough development or vital hints to justify the theory.
Anyway, if we assume that the dream theory is true, it just seems too elaborated and complex, considering the dreaming sequences that Squall and company had about Laguna (a dream within a dream, lol). And come on, I doubt anyone would have dreams regarding a random sorceress threatening to destroy the world with time compression. Heck, it's been years since I played the game and I still don't understand that part! They never even revealed her motivation to begin with! That said, perhaps reading the article will change my mind.
If this theory turns out to be true, then it's kind of sad that Squall's moments with Rinoa were all but fabricated pieces of his dreams. ><