No. I don't know where you took your definition but it's incorrect/incomplete.
A nerf is something added to make a item, character or similar WEAKER. A situation like you describe is a nerf but also a author adding or removing abilities that the character could have used. A character can even be nerfed after a incident when a author reveals details that it could have used. This definition applies not only to mangas but games.
In this specific situation BM was nerfed by the plot (and both Law and Kidd were buffed to be able to stand to her) and didn't use abilities that we know that she has and uses normally. Basic example is her ACoA that she used with Kaido from the start and was strong enough to almost destroy the island and that she forgot during the battle.
A nerf is something added to make a item, character or similar WEAKER. A situation like you describe is a nerf but also a author adding or removing abilities that the character could have used. A character can even be nerfed after a incident when a author reveals details that it could have used. This definition applies not only to mangas but games.
In this specific situation BM was nerfed by the plot (and both Law and Kidd were buffed to be able to stand to her) and didn't use abilities that we know that she has and uses normally. Basic example is her ACoA that she used with Kaido from the start and was strong enough to almost destroy the island and that she forgot during the battle.
