Oh, they sort of explain that in the introduction. They admit they are going off of a more broad definition of "noir." They provide these three quotes to give you an idea of their criteria:
"Film noir is not a genre....It is not defined, as are the western and gangster genres, by conventions of setting and conflict, but rather by the more subtle qualities of tone and mood. It is a film "noir," as opposed to the possible variants of film gray and off white." - Paul Schrader, Notes on Film Noir, Film Comment, 1972
"Film Noir is a historical, stylistic and thematic trend that took place primarily, but not exclusively, within the generic complex of the American crime film of the forties and fifties. The term was first introduced by French cinéaste Nino Frank in 1946. For many years it was known only to the French, who seemed to be the only ones equipped (critically or otherwise) to grapple with its definition and/or historical implications." - Spencer Selby (Dark City: The Film Noir; 1984)
"Film Noir is the flip side of the all-American success story. It's about people who realize that following the program will never get them what they crave. So they cross the line, commit a crime and reap the consequences. Or, they're tales about seemingly innocent people tortured by paranoia and ass-kicked by Fate. Either way, they depict a world that's merciless and unforgiving." - Eddie Muller