The points I can remember from the article that really make this theory credible are:

*Ultimechia just happens to "summon" Griever.. the same lion on squalls necklace.

*Everything in the flashback at the end of the game is from disc 1 except the rinoa in the space suit.

*Squall gets Rinoa even though she showed interest in only Seifer throughout disc 1

*Some of the events after disc 1 had no hinting to in disc 1.

The only thing in the game I can think of that doesnt, 100% confirm this theory for me is when Irvine says on the train to Deiling, "Maybe its Fate". after "re-meeting" with Selphie.

Magnificent Theory by the author though.