Oda didn't have to make her that strong if he wanted her to join the crew w/o upsettin the power ranking. And he doesn't mind makin chars appear weaker than they rly are when it suits the story, he did it many times.
Also, I don't see how she'd have made the battle w/ the Seraphim particularly easier if she doesn't know abt their weakness. Zoro knows yet they still struggle.
I think Oda just doesn't want her to join the crew and the fact he made Kuzan the 10th TC's a clear indication of that imo. Doubt Oda sees the story through the prism of power lvls like the fanbase does.