No offense, but the idea that authors prioritize plot over power level stuff is not really a groundbreaking revelation. It's kind of obvious.
It doesn't really matter what level on Oda's list it is as a priority, it's still what's relevant when we are discussing the levels of characters. There is no source we can use beyond fiction. For the time being, there's no reason to think (Young) Garling isn't weak, since this is the only real fight he's been in the entire story. No other character on GV has been treated this way, do you think someone like WB would get neg diffed like that? Ofc not.