There is something more to Dieselpunk, though. It's seeing the future through the eyes of a specific time in the past.
Steampunk is set in the past, but most Dieselpunk I have run into is set in the future relative to the specific time in the past you are looking forward from.
For instance, if we look at "Sky Captain and the World of Tomorrow", it's set in 1939--but a 1939 as might be imagined by someone in the 1920s.
Likewise, you look at Terry Gilliam's Brazil--it's set in a dystopian future as might be seen by someone in the 1960s, during the rise of technocracy, as viewed as a breakdown of a technocratic system into a nepotist's wet dream.