... from the narrator that Lizzy doesn't have. We are not entirely dependent on seeing Darcy's actions and drawing conclusions about his underlying character based on that.

The narrator has told us that Darcy is haughty (proud) and that he has dismissed the entire roomful of people as uninteresting without meeting them (prejudiced). Even if (and I'm not entirely convinced of it) Lizzy should have reserved judgment and taken into account that Darcy might merely be shy (or whatever), we don't need to. We have the word of the narrator.

I agree that we shouldn't necessarily view every action of Darcy through the knowledge that he is both proud and prejudiced, but it seems to me that to ignore those characteristics is merely the other side of the same coin.