I think Oda purposefully had her down while he used WSG, WSG looked very similar to the move she used on Alpha after all, and G5 in general is pretty similar to how Nika is described in storybooks.
If he uses a move like that again on Saturn then she'll definitely realize he's Nika.

Also @Tensub thoughts?
That's why it has to be Nika who saves Bonney from Saturn, this goes very deep and it concludes perfectly the flashback in a very good manner as it started with Bonney asking if Nika will liberate her too.