I think this is what distinguishes modern non-fiction from non-fiction in earlier ages. In earlier ages (Melville’s time), the non-fiction took pains to exclude any fanciful information at all. Non-fiction was devoted the presentation of actual evidence that supports a particular conclusion or theory. Fiction (such as Moby Dick) was a separate work that incorporated those facts into an illustrative example. Today’s non-fiction typically incorporates both fanciful illustrations and actual facts. The danger is that some readers may confuse the illustration as another fact.