There is indeed a considerable amount of focus in the book on the idea of, and belief in, a black/white dichotomy in America, as in the tendency to define white as "not black".

I didn't get the sense that Painter was implying with this focus that the black/white dichotomy is "real" (except in people's minds), or that it is legitimate not to consider Americans who are neither white nor black. She's studying the prevalent white belief that this dichotomy is real, and the mindset that leads to such exclusions, where blackness looms very large in how whiteness is conceived of. Analyzing the thought process of "Thank god I'm white, thank god I'm not black, other options are irrelevant" is a large part of the book, and there is some discussion of attempts to shoehorn different racial groups into this already tenuous white=good black=bad dichotomy.

I'm not sure if I'm explaining this very well or if I'm really answering your question. I guess to me it seemed that whenever the discussion was narrow, it was because she was talking about belief systems that are narrow, not because the author's own beliefs are.