At the moment, I regard retro-noirs as a subset of neo-noirs. The latter are defined on the web as follows:
"Neo-noir (English: New-black; from the Greek neo, new; and the French noir, black) is a style often seen in modern motion pictures and other forms that prominently use elements of film noir, but with updated themes, content, style, visual elements or media that were absent in films noir of the 1940s and 1950s."
I find this definition needs a bit of work. I would say that the neos place a different emphasis on themes, content, style, and visual elements that are present in classic noirs; and, in addition, in some cases they originate new elements, while sharing classic noir elements. I think that's more accurate than saying they update with stuff that was missing from classic noirs. By this definition, neos are not simply later noirs. While still sharing some classic noir features, they are different enough to set themselves apart.
I'd argue that retros belong to the broader class called neos by virtue of being new noirs or modern noirs (beyond the classic period) and, like other neos, of possessing thematic, content, style and visual elements that differ in emphasis from classic film noirs. If that's correct, then retros should share certain features with neos but they should also have some other elements that set them apart as a subset of all neos. What are the latter that are shared with the other neos, and what are the latter that set them apart from the rest of the neos?
Well, maybe someone else would like to address that. I haven't thought about it much. It may be enough to say that a retro is simply a neo that's set in an earlier era than when the movie is produced. The current ones called retro are in the 30s-50s. As time goes on, maybe they'll be set in the 60s and 70s, etc.
Somehow, I don't think that's all that distinguishes a retro. When a movie is set back in time, a great deal goes with it in terms of costume, language, gadgets, social-political background, appearance, sets, buildings, cars, etc. The story in a way writes or makes history. It changes how the audience perceives that era and those people and then themselves now and their current situation. It lends perspective. Themes might be explored using the past that would be difficult or sensitive to explore in the present.
Certainly, a retro gives me a different feeling than other neos. Why is that? One reason is my age. I feel more comfortable in the older worlds in these movies. Things are simpler and slower-paced, and that reflects in the ways that the story is told in the movie. The retros, as far as I can remember, don't have the sometimes hectic cutting of other neos, the jarring rearrangements of time, the shattering of personalities, and the sometimes outrageous behavior of people (like crashing cars or intentional mutilation). The characters relate to one another in different ways too.
I might have a look at Delusion (1991) because I have it and never watched it. It's been some time since I saw White Sands, which I also have. I remember liking it quite a lot.