Also, 2.5 W/kg in a shielded room is far, far more than 1.6 W/kg exposure. Cell phone radiation, when it even goes through your head, keeps on going right through to the other side[1]. In a shielded room, the radiation bounces off the walls until it's absorbed by the stuff in the room... ie the rats. The absorbed power in humans is microwatts. The rats were absorbing fractions of a watt.

[1]: as evidenced by the fact that cell phones actually work. When you cover them up with your hand, you don't suddenly drop the call.