Simply put, because the beings that Eorzeans refer to as Primals, are spent. I think it's safe to assume that, at this point in the story, we have seen all of the Primals Eorzea has to offer. There are no other beast tribes that we know of, and we have fought and killed every Primal in Eorzea at least twice, sometimes thrice. So, when that little spawn says "I speak not of Primals, with which you are exceedingly well acquainted" he's basically saying, "oh, not Shiva, Ramuh, Ifrit, Titan, Garuda, or Leviathan. We're done with them." It's basically a declaration that the story is moving away from Eorzea, which is why Odin is such a good choice to bridge the gap.
Odin has been in Eorzea, (and even titled an "Elder Primal" in the local lingo), but he's not from Eorzea. His origins are from the Allagan Empire (and much further North, if we follow his lore). So, he is the first of a new breed of Primals that we will be dealing with as the story takes us further away from the country of Eorzea - to lands unknown, with Gods that we do not know, have never met, or, in some cases, have never even heard of. In the case of the Garlean Empire and the Allagan Empire (regardless of how different those two Empires are from one another), they both refer to "god-like" beings as Eikons, not Primals. In the Garlean's case, this has been proven by the 2.0 story line when Van Belsar refers to the Primals in Eorzea as Eikons as well. He does not distinguish between Primals and Eikons. They're one in the same to him. It's just a different word. Which means that any "Primals" we run into in the Garlean Empire (if we ever go there) would be referred to by the locals as "Eikons."
It's true that we can't make the same claim with the same certainty for the Allagan Empire, but, as of yet, it doesn't seem like it would be an outlandish assumption. Honestly, given what we know about the only "God-like" beings we've actually met that had anything to do with the Allagan Empire (Odin and Bahamut) they seem like Primals. Super strong Primals, sure, but Primals. It would seem that the Allagans just used the word Eikons, and that that spawn is using the same term to refer to Allagan related Primals that we have yet to meet.