In Redemption, Guinan tells Worf that she made a bet with Picard that she could make him laugh before he became Lt. Commander. Worf says Klingons don't laugh. Guinan says yes they do, and that she's seen him laugh.
Wrong, not seen,
him laugh in Yesterday's Enterprise. You already won the bet.
She probably means more of an actual, genuine, laugh. The laugh he makes in this episode seemed more of an intentional, ironic, laugh.