In the line just above SW doing the "very gallant thing", Darcy responds to SW with his feelings about dancing, "It is a compliment which I never pay to any place if I can avoid it." SW I am sure was aware of Darcy only dancing with the ladies he knew at the Meryton assembly. Perhaps SW thought he could facilitate things. Although the snub gave Lizzy and Charlotte something to laugh over, I suspect SW did not think it so serious an offense since this was the first evening they had ever seen each other.