I think it is Tyr, that establishes the Link into the other expansion. It would be a bit weird for Iridrikon to just appear, say some villain lines and then disappear again (but it would be very LK in WotLK-like)
I think it is Tyr's story that establishes the ongoing story, past DF. It could be that the Discs which we try to retrieve from the Nighthold which hold the memories of Tyr (and not some edited records) reveal the other side of Azeroth to us and how to get there.