To be sexist implies that you consider all of the people of one sex to be a particular way, weaker, vainer, or simply different in some particular way.

I have to strongly disagree. To use an extreme example, say there is a man who says, "I think all women in the world except, say, my mother is a vain, technologically inept idiot who has no value except as a receptacle for bearing children." Does not having that misogynist opinion about one person make him any less of a misogynist? No.

Yes, an accusation of misogynism has to be based on a general trend in someone's behavior, not a single isolated example. But Susan is not, by any means, a single isolated example. Consider what synaesthete7 herself has said about "The Shoddy Lands"; consider what other posters here have said about the portrayal of women in other works of Lewis's fiction, perhaps most strikingly in That Hideous Strength.