To start, there are separate weekly lockouts for 10-man and 25-man, allowing a guild in Asia to run both in the same week. Secondly, the loot in 25-man is 8 ilevels higher and the boss health and damage is also 8% higher. This allowed Tianqi to acquire higher ilevel loot in 25-man before going into 10-man to achieve their Garrosh kill. At least one of the players participating in the kill had a average ilevel of 575 along with a ilevel 588 weapon.
These circumstances should in no way detract from their achievement, but the traditional world first race is still very much on for both 10-man and 25-man.