I agree.
For what it's worth, Edmund makes basically the same point to Fanny earlier in the book, when he says that Henry could not make Mr. Rushworth unhappy if Maria didn't let him. Miss Manners is constantly pointing out that there's a natural tendency for the aggrieved party to blame the interloper (whom they wouldn't care about otherwise) rather than the spouse (whom they do love and care about) but it doesn't really make logical sense.
For what it's worth, Edmund makes basically the same point to Fanny earlier in the book, when he says that Henry could not make Mr. Rushworth unhappy if Maria didn't let him. Miss Manners is constantly pointing out that there's a natural tendency for the aggrieved party to blame the interloper (whom they wouldn't care about otherwise) rather than the spouse (whom they do love and care about) but it doesn't really make logical sense.