You know Moose, the more I think of it the more I think the mysterious Primal that so put the fear of the gods into the XIVth shortly after the fall of Ala Mhigo was just a mislabeling of Midgardsormr on SE's part, as they called Midgardsormr 'an elder Primal' on the 'Waning of the Sixth Sun' recap disc on the Collector's Edition, only to then state that was a mistake.
So it's likely the labeling of a 'Primal' appearing to halt the Garlean advance is just another typo in the same way (it fits the time frame for the supposed incident, and also remember that during the Battle of Silvertear Skies, when the Agrius crashed and ruptured the seal in Silvertear Lake, Primals in fact did appear - but not just one, hundreds of them shot out of the aether at high speed.).
