Just because something isn't shown in game doesn't mean it didn't happen in the overall story. Disagree with that if you want to.If it doesn't happen in game it doesn't count. Why? Because Blizzard have "forgotten" the out of game lore plenty of times before.
The Horde can't be shown weaker because they'll need the same in game presence for quests and flight points for example. The Horde won't be kicked out of Pandaria despite the damage they've done because that wouldn't be fair.
And that's leaving aside the trustworthiness of said tweets. It wasn't that long ago that the Blues came out with tweest and so on telling us the Alliance would be getting story in MoP, they'd have their moments of glory, their times of fist-bumpiness.
Do you honestly think getting roped into the Hordes Rebellion storyline counts as lore progression and development for the Alliance or any of its factions? That Anduin going neutral and becoming a Horde apologist helps showcase the Alliance? That having Alliance players beat up shopkeepers makes for a fist bump moment of glory?
Blizzard obviously thinks so. A lot of players - many of whom are neutral/play both sides or are even Horde players - think otherwise.
So - if the Horde does come out of this "weaker"....but with the same in game presence as the Alliance, the same number of flightpoints, the same quest goals with the same ability to progress their anti-big bad agenda next Xpac and no loss of territory and a peace treaty that recognises their acquisitions....would that count as "weaker"?
What about if they withdraw from Azshara? Does that count if a later story then retcons that because the author "forgot" as it wasn't in game? How many times did Varian have to get cured before it became lore as one example - simply because it wasn't shown in game therefore didn't exist for the various writers?