There's always going to be a soft period between styles. People don't suddenly dump their mindset overnight.
Take steampunk for example. Everyone says "Victorian Victorian Victorian," but Victoria died in 1901. Between 1901 and the end of World War I, there was a soft period (Edwardian) where the aesthetic changes from steampunk to dieselpunk.
If people want to think of greasers as dieselpunk, then I have to disagree, but I can see their point of view.
To me, rockabilly, greasers, post-WWII style, and what we commonly think of as '50s culture is atompunk territory.
Personally, '69.Manned spaceflight plus 2001equals Buck Rogers getting smashed to bits.