They were not called the "WG's greatest military weapon" but the "
NAVY's greatest military weapon".
The Navy = MARINES
The World Government = GOROSEI ruling over the world
World government > Marines
Imu > Gorosei > Admirals
Admirals are the "ultimate assault force" by the World Government because after them, there is nobody left but the Gorosei.
Aokiji was introduced in chapter 303. Gorosei were introduced in chapter 233. Oda planned the Gorosei before the admirals.
Admirals are the most powerful officers of the NAVY.