Personally the thing that turned me onto this theory the most is something that I don't see brought up very often.

When doing the Zodiac Weapon Quests back in 2.X, as we conduct research into the Zodiac Weapons and the Zodiac Braves that used them, a point is made that such Legendary Heroes have been noted as appearing throughout history during times of great strife, with the Warriors of Light evidently being the latest incarnation of such. The idea of cyclical appearances, almost as if in response to the prayers of those who grew up on tales of such heroes, just feels very Primal-like.

Compounded with the declarations of utter disbelief and/or outright horror from pretty well every confirmed Primal we've fought, and it certainly becomes intriguing at the very least. I really do hope to see the writers lead us down this path a bit more directly, even if just to dispell the idea in the end.