Bosses don't have 13,083 armor anymore.
Instead, they have 10,643 armor. The implications are:
1. This is a straight-up buff for any group that does not have access to the Major and Minor Armor Debuff categories
2. Any group that does have those debuffs will see no change, as the armor reduction is the same given that the armor debuffs are now a percentage value rather than a flat amount.
This post in the Combat Ratings thread clearly shows Sanctum Sentries (Auriaya's adds) in Ulduar having 10,643 armor.