So how do we know that
Ulfar, specifically, individually, did not join the Kul Tiran society? I'm asking because, as far as I can tell, the
only reason to say that he did not is solely to support this "'their' descendants = drust descendants" claim.
I've read and re-read his WoWpedia page, along with his quotes. Nothing in there even hints at him, specifically, never having joined the Kul Tiran society. So that means we basically have an assumption without evidence being used to support a claim.
We also have Gorak Tul angry at Ulfar for teaching the secerts of the Thornspeakers to the Kul'Tirans,
with Gorak Tul calling the Kul Tiran "wretches". However, I'll freely admit that this is not conclusive evidence of anything, since his words could go either way, meaning that Gorak Tul disapproves of teaching the Thornspeaker secrets to non-drust, or that he simply considers the drust descendants "abominations" if the Kul Tirans are indeed a mix of drust/human blood.