The thing about Film Noir is that studios and filmmakers in the 40s weren't aware that they were working within this genre, it was a term applied by French film critics and not embraced by American filmmakers of the 40s and 50s. As far as they were concerned, they were making dramas.
Chinatown is a self-aware, revisionist noir, consciously using the conventions of the genre, so it really cannot be placed in the same category as 40s and 50s films. Also, the films considered to be the first noirs are "The Maltese Falcon" (1941) and "Stranger on the Third Floor" (1940) - which means that Chinatown, set in 1937, takes place in a pre-noir era. The height of noir happened in the aftermath of World War II, and those films expose the dark side of the post-war prosperity. Chinatown, by contrast, is made in the aftermath of the Vietnam war, and it looks back four decades to create a narrative of corrupt institutions, and about the futility of good intentions that is more of a response to Vietnam, in the way that the classic noirs are deeply tied to World War II.
So, I think it's a different dish.