Ion said that the Scourge drew a most of the Scourge forces from Maldraxxus, but with 'most' it's not all. So my guess is the guys looking like they come from Maldraxxus come from there, the guys looking like Revendreth come from there, which would mean the San'layn. The Lich King may have just used the souls that originally went to those places and maybe even picked the ones he wanted, which would also explain why he had people that he had already known in life.
But that is just speculation of course, I hope we find out more once we get to the Shadowlands ^^