Because of the stat squish they were aware of the possibility of this happening but added code to specifically prevent it.

http://us.battle.net/wow/en/blog/15682579/#stat_squish
It's important to understand that this isn't a nerf, and we have special handling in place to preserve players’ existing ability to solo old content. Players will deal bonus damage against lower-level creatures from past expansions, and will take reduced damage from them.
Maybe they did just the mobs, and forgot to reduce the bosses.