Of course, but that doesn't take away on Broken Shore there are eredars summoning void creatures, and also right next to that area other eredar warlock npcs that use void walkers as temporary minions.
Also KJ has a phase where he summons void creatures.
Priests class is more void-lore then ever before this expansion, of course we knew a lot about the naaru light/void balance since TBC and WoD, but Legion made it even more personal for players.
So even if it's not Sargeras directly using the void, at least KJ's invasion forces do. Maybe KJ went mad too and got whispered by void lords as well to oppose sargeras indirectly... I dunno, we'll see...
Maybe it was all a trap to lure us somewhere, KJ even admired Velen's powers and visions at the end.
For the story it makes sense to put the most powerful players & npc characters on like Argus temporarily, so the Old gods (N'zoth) can come out of hiding and strike for 8.0.