After doing some research, I believe we're both right, in a way, and both wrong, in others.
While the information you bring up is interesting, the description of http://us.battle.net/d3/en/item/immortal-kings-eternal-reign
says "Worn during the Immortal King’s final battle." This could mean that Worusk is *the* Immortal King and wore it during his final battle (meaning Worusk is another name for Bul-Kathos), that Worusk is *an* Immortal King and wore it during his final battle (meaning the title Immortal King applies to more than one person), that Worusk is *an* Immortal King and did *not* wear it during his final battle (meaning some other Immortal King wore it during his final battle, possibly Bul-Kathos), or that Worusk is *not* an Immortal King and that the item was worn during the final battle of a person known as the Immortal King, most likely Bul-Kathos. I bring this up because it is not only important, but it establishes that Worusk was either *the* Immortal King, one of two or *more* Immortal Kings, or that at least one item in the set predates Worusk if you believe him to be a later Barbarian Chieftan.
states that "In the days when the barbarians roamed the entire western continent—long before the coming of Rakkis—one man, Worusk, united the tribes, and for the briefest of moments, the barbarians had a king." Rakkis founded Westmarch, so Worusk predates Westmarch. Worusk united the tribes when the barbarians were roaming the *entire* western continent, meaning they weren't limited to the Northern Steppes. The continent (pre-Worldstone explosion) extends down to where Kingsport is currently located. To the best of my knowledge, there have been three times when barbarians might have roamed down there: Now, which is **post**-founding of Westmarch, shortly before D2, which is *also* post-founding, and before Bul-Kathos created the Vigil. So, as Worusk's actions are pre-Westmatch, is must be pre-explosion and and pre-D2.
If the barbarians did not roam after the Vigil was set (and, to the best of my knowledge, they *didn't*, as they became an isolationist people), that could mean that Worusk is actually Bul-Kathos. Meaning I'd be right in saying Worusk, the king of the tribes, and you'd be right in saying Bul-Kathos.