Lass, I agree with what you're saying, but IM invented that narrator.
He is saying this is what goes on in the head of this woman. This is the kind of book she would write.
And that narrator is an unpleasant and disturbed individual.
I think she is unpleasant and disturbed in ways that people view as stereotypically female, so he has written a sexist book. Other may disagree as to whether or not she is a sexist stereotype. If she's not a stereotypically 'bad' female, merely unpleasant and disturbed, the book is not sexist.
I do think that people are interpreting it as being seen from the perspective of multiple characters and what they think or do, but we only see/hear it from one perspective - Briony's. She is the narrator and we never know if she is saying anything truthful about 'real' people.
The only bit that sounds truthful is near the end when one of the twins cries, and she can't understand why. That then seems ironic, that we can understand the meaning of that and she can't. But even that could just be her being 'clever.' Maybe within the context of her story, she made the twins crying up to create an ironic moment, maybe the twins didn't exist.