I have to say I agree with you, although I don't necessarily think the difference lies specifically in the areas the book touches. However, saying there is no difference between, let's say, a female INFP and a male INFP is denying reality. I know that when I'm "classifying" my observations about types in my head, I always subconsciously create a divide between male and female representatives, because the females have certain tendencies that the males just don't share and vice versa. Of course they're similar in many aspects either, but I'd say the mental connection I have with them is on a different wavelength anyway. Finding a girl who is "just like me" is much easier than finding a man who is, and not because of type (as a matter of fact I know many male INFPs and they'll
always have a more "T" vibe than INFP girls, for example; and also many shared traits that the INFP girls can "understand" but they'll still annoy them
).
I'm not sure what is the basis of this difference, and perhaps no one can tell with certainty. My boyfriend believes that basically, "all men are concrete and all women are abstract". Yes, it's a stereotype, but if you look outside the "living in your head" stuff and cognitive functions, it might be something to think about.
For example, a friend of mine read a book (unfortunately I don't recall its name) where the author presented an interesting metaphor (I haven't read the book so I might twist it a little, but this is the general idea): all men are kind of standing on the shore and watching the boat where their dreams are in the distance - if it comes, wonderful; if it doesn't, they'll wait some more. On the other hand, women do not watch the boat; they create the boat, and if the reality doesn't match their dreams they'll take a new approach to reality and recreate it (if only inside their heads) so that it matches their dreams better. I think it's really spot on, and I'd say it correlates with the "concrete/abstract" very nicely.