What's there to understand is, Zoro expects his captain to be at a level if he wants to be above him. That's not the same as I don't want to get stronger to remain beneath my captain lmfao. This shit is so basic. I too am not sure why it's so hard for you to grasp.
And that level Zolo expects from Luffy is much superior than his own level. Thus, you can compare the strongest pirates such as Luffy to the strongest Marines who are Admirals, Admirals are the counterparts of the strongest pirates who are called Yonko or Great Pirate etc. Not counterparts of strongest subordinate pirates. While its obvious
strongest pirates > strongest marines for the story that is for pirates, strongest pirate subordinates being > strongest marines would be too much even when I make fun with Ladmirals and Ladmiral fanboys, LOL.
At best, strongest subordinates can be equal to Admirals or when Admiral joins those pirate crews, Aokiji immediately becomes one of the strongest pirate underling ever. EoS Zolo will need extreme extreme diff vs an Admiral, it could go either way a close battle.