In that case, he wouldn't have had the homies on her fists, a technique she uses for punching, and Brook wouldn't have had a broken bone. But it's the same, in reality: BM with her homies didn't obliterate Brook on the spot ad one of her Homies even got damaged, and only because of the plot.
In this manga, everything is "because of the plot". Characters are strong or week or idiot because the narrative says so; all of them