I believe they are still self-interested when making judgments however because the author is implementing an ignorance to their own "self-interests" they don't know what to be self-interested for. So it's not that they are setting aside their own self-interests to make fair judgments; rather, they don't know what to be self-interested towards because of the veil of ignorance. With the cake example, the child cuts the cake equally in self-interest because he doesn't know what slice he's getting and doesn't want to be stuck with the smaller size. I don't know if that makes sense but that's how I perceived it.

interested!