That is just a random event. Actually, one of 2 random events that appear identical to the player. One has Minister Koun join as a second copy of your civ in a permanent alliance with you. The other has him enter the game as a second copy of your civ with no special relationship with you at all. Neither one makes him a vassal r have the added diplomatic bonus for liberating him.
Soon, that event should pick civ specific minor leaders.
I was really annoyed when I had this event happen to my permanent ally Tasunke in The Cult scenario. Minister Koun entered a a seperate civ whose territory included most of the Pass itself. He could not be convinced to give be open boarders and the game options did not allow me to declare war on him, so the game would have been almost unwinnable if I hadn't cheated. (I guess I could have tried sending only Nightwatches and Shadows though the dungeon, but I don't know if that would have been enough. I didn't have access to war-causing spells yet, but I think I heard that they no longer work in Always War or Peace games anymore anyway. I still consider that to be the one time I didn't cheat, even though I did use worldbuilder to make Koun my vassal. Such an event should not make victory impossible.