Well yeah, Akainu and Kizaru weren't born into their position, Garling was.
Garp,Roger and Rocks were all stated to have been running the era 44 years ago iirc, when at that point Garling had nothing to his name. Roger and Rocks have fame from even before that, with Brook recognizing Rogers name from iirc, 50 years ago ish, and Rocks starting his career after killing an admiral 57 years ago. Garling has never really been someone mentioned as among the old legends either. He could have peaked in more recent years, even if he did allegedly scar WB(which I don't really believe, but for the sake of argument) it could have happened many years after GV.
That's not true, because not all characters peak at the same age. Even if Garling was Roger/Garps age, it's possible that losing to Rocks is what inspired him to train more and reach top tier level, when previously he was just thinking he was the shit like any other CD. Plus, the difference between Garling and those other characters is his role in the story isn't done yet, while the rest won't really be in any fights aside from this FB.