If you look at Sky Captain And The World Of Tomorrow, you can see very easily that steampunk is just a subset of some larger category that Sky Captain is also part of. That category is revisionist sci-fi. If you expand the category further to fantastical revisionism, it suddenly includes everything J.R.R. Tolkien ever did (and, for that matter, The Flintstones).
In practical terms, many more people in our society get their information about the past from fantastical revisionism than from actual history. I think this is probably because the semiotic functions of stories about the past are more powerful culturally than the literal record of events, and semiotic purposes are actually better served by a medium which does not require literal accuracy.