Hmmm. I think that's an editing mistake. Originally, Wandering Xykon was going to be supported by both, just like original Xykon. But Goblins are so populous that he ended up sucking up piles of 5-10 enemies at a time.
Well, you can either play it so that it's just Undead and ignore that one line, or you can have Xykon carry along a bunch of Goblin sycophants that don't actually help him. It would certainly serve to break up any large goblin stacks along the way, which would in turn help the game go even faster.
But our final intent was to keep it only Undead.