Here’s the incident:

“‘Which do you mean?’ and turning round he looked for a moment at Elizabeth, till catching her eye, he withdrew his own and coldly said: ‘She is tolerable, but not handsome enough to tempt me; I am in no humour at present to give consequence to young ladies who are slighted by other men. You had better return to your partner and enjoy her smiles, for you are wasting your time with me.’

Mr. Bingley followed his advice. Mr. Darcy walked off; and Elizabeth remained with no very cordial feelings toward him. She told the story, however, with great spirit among her friends; for she had a lively, playful disposition, which delighted in anything ridiculous.”

Our characters are nerds. Geeks. They do this all the time. When he says “You know how I detest it” she knows he’s referencing Pride and Prejudice and the inauspicious first meeting of Elizabeth and Darcy. That’s why she smiles and why she’s drawn in.