Steampunk is in essence an immitation of Victorian era fiction so I think calling original Victorian or early 1900's fiction works "steampunk" - like 20,000 Leagues (originally written in 1869) - is kind of an odd viewpoint.
It would be like creating a new style of art based on cubism, naming it something... "cubepunk" and then referring to an original Picasso cubist painting by that description. It might be fun but I, at least, wouldn't view it as accurate.
I'd also disagree with the notion that Dark City is steampunk personally... It does have a bit of an "out-of-time" quality to it but beyond that, I wouldn't call it Victorian or see any evidence of psuedo-victorian-age technology. It's one of my favorite films, regardless.