I have two wildly insane theories for you tonight, both of which are subject to change (or the trash bin).
Theory 1: Humans ARE Native To Tyria
They just saw the wisdom of fleeing ASAP a few dragonrises ago, and encountered the Six in another dimension/planet/whatever. And by the time the Six felt the draw of the magical power of Orr, a few millenia had passed and there weren't anything but a few legends left of the land of the dragons. So they came, and since Tyria loses it's own history every dragonrise, we think they're not native to Tyria.
TL;DR - they left and then came back. They were on extended vacation. Just because they arrived doesn't mean they never left.
Theory 2: Norn Originate From An Ancient Human/Kodan Pairing
If you mix a giant bear and a human, what do you get? Norn are just big humans. They have some kodan in them: they both have some version of Spirit lore, for example, and norn can shapeshift with the blessing of said Spirits. Significantly, humans who believe in the Spirits are implied to not gain the ability to shapeshift, so being norn (or having kodan blood) is a prerequisite for this.
If humans were around a long time ago, and no clue how long the kodan have been around (they have creation myths, which honestly very few Tyrian cultures have), there could easily have been enough time for the norn to multiply into their own race.
Maybe the human/kodan pairing violated some ancient kodan law and that's why they don't acknowledge (and in some cases outright denigrate) the norn. It would also explain the VERY humanoid appearance of the norn, which literally no other race has. (Not counting sylvari; they're MORE recent than humans.)
TL;DR - it perfectly explains why Anet sometimes treats norn as just 'big humans.'