Kong is the supreme commander of the WG's military forces. Yet Garling is the supreme commander of the holy knights. The holy knights aren't simple military, they are the highest ranking Celestial Dragons. Dragon said the real battle only begins once the holy knights are dispatched, which speaks for their significance.
Furthermore, the 3 admirals as a group were called the WG's strongest military force, with Aokiji being "one of them". Now each of the admirals is a bonafide top tier, so as a group they might as well be superior to the holy knights as a group, given most of the holy knights are probably going to be beaten by Revo commanders or some straw hats. Shamrock is probably carrying the entire team more or less as the only top tier. So the admirals as a collective being supposedly stronger doesn't translate to an admiral being able to beat Shamrock 1 on 1.