Highlighting the relevant bit for you. One cannot retcon an assumption. We just assumed incorrectly.
Semantics and pedantry aside, I don't think it's a retcon. Players have always assumed that when we were fighting Old Gods, we were fighting what is essentially their big toe or whatever. It's always been thought that they are much bigger and much more powerful and terrifying than what we face on the surface. Void Lords are basically that. Old Gods are manifestations of the Void Lords because the Void Lords cannot exist in the WoW universe without exploding or something. So instead of fighting the Old Gods' big toe, we've been fighting Void Lords' big toes. Blizz probably just gave them a name to categorize them as being on a whole other level than the servants we fight. And Void Lords WOULD absolutely wreck Sargeras. So the lore hasn't really changed at all, we just got a name for that unknown 'full power' version we've always supposed had to exist outside of Azeroth's dimension.
Okay, you're Blizzard. You're Kosak or Metzen or whoever is in charge of lore these days. When would you have told players the pantheon is dead? How? It's been foreshadowed ever since MoP at the very least when Wrathion ate Lei Shen's heart and had his titan freak out, so clearly not at that point since they only wanted to tease it a little. The end of the expansion, we were fighting orcs, hardly the time for Titan lore. Then we jumped straight to WoD, where the entire expansion took place in an alternate universe where there was zero room for Titan lore whatsoever. Where would you drop that lore bomb in WoD without it being shoehorned in? Or would you, and follow me with this one, the logic is complicated, bring it up in an expansion devoted to the Legion where we learn more about them and Sargeras and have entire zones and dungeons dealing with the Titan constructs they left behind? You're also churning out this Chronicles book that details the ordering of the Warcraft universe and goes more indepth into the history of Azeroth, the Titan Watchers, and its native races than any other source before it. You could just omit the part where the Pantheon are murdered and the last of their powers are sent out to the Watchers, but in a book that's meant to fill in holes in the story, why make more? The lore nerds will have something to discuss, and the people who only play the game will learn about it when its relevant to the current day story.
tl;dr I'm sure it'll come up whenever we deal with Sargeras if it's not already in Legion somewhere already.