It's no stranger than any apparent battle between Rinoa and Ultimecia (assuming one is an advocate of R=U), although S=G is inescapably wrong. An examination of the Japanese dialogue makes it very clear that Griever was Drawn directly from Squall's mind, which would obviously disprove the theory. In general, most S=G proponents use the idea in a misguided attempt to further the R=U argument.

EDIT: Ultimania Guide =