Raiders have been pretty used to seeing zone wide nerfs and buffs on current raiding tiers once the main progression race is over and less progressed guilds are still struggling to beat bosses. Blizzard usually gives a raiding instance a zone wide buff, such as the DPS buff we saw in ICC or a zone wide nerf to boss health and damage, such as as we saw in Dragon Soul.
But even though this is an expansion ending raid tier, Blizzard confirms we will not see the same buff or nerf for Siege of Orgrimmar. Instead, they plan to look at progression and make targeted nerfs instead.
The tweet was actually made last month, but MMO-Champion recently highlighted it, as it was something many people missed.
This will also likely make the market for heroic Garrosh mounts last longer since fewer guilds will be killing heroic Garrosh than might have done with a buff or nerf to the instance, which is good news for those who are killing heroic Garrosh easily enough that they can carry an extra body, but bad for those who were hoping for a nerf to be able to kill the boss.