That big red thing that disrupts the Arbiter is a huge surge of Anima sent by Denathrius. He's been the "inside man" all along.
Argus had nothing to do with it.
We also don't know how the timeline works in the Shadowlands, we still don't know how souls work exactly - like the cinematic implies that Uther went up right at the moment of death, but we know his ghost still helps us in the Halls of Reflection which implies that the part that helped us was the wounded part of his soul that was sheered off by Frostmourne.
Basically there's a long game at work, but I'm not sure the "death of Azeroth's world soul" is the endgame yet either, I mean it might have been mentioned somewhere in the game but not that I can see yet.
So we're still waiting to see what the Jailer's true plan is outside of the nebulous concept of "freedom."