I know how mentally slow you are but this is sad, nowhere in there does it say they are a secret, it only says that they rarely come down to the ground.
Just like Gorosei aren't secret, they just rarely come down that no one has seen them.
Bet your dumbass also believes that Yonko are a secret just because Nami just learned it.
Garling wouldn't be able to give a command to marine fodder if marine fodder didn't know who the fuck he was, do you comprehend that, schizo-chan?
Marines know who these bums are, and marines also know that Admirals are the strongest force.