Well for one we know that even amongst captains, Coby is superior, as he is a part of SWORD, a special secret organization.
but more than that, rank does not equal strength. Part of the reason why coby’s rank might not be that high is that he isn’t as active as other marines. He still trains a lot with Garp and hasn’t been on many missions.
In any case I think it’s dumb to suggest that from the time of fishman island to now, Coby has made Zero progress in terms of strength, especially considering how much he grew from east blue to envies lobby.
training with garp is no joke. After Amazon lily I think he will at least be RA if not VA