Mr Darcy's involving Elizabeth in the conversation may be merely an act of good hospitable manners, as she's just approached the card table. That'd be of a piece with this later incident (ch10):

The path just admitted three. Mr. Darcy felt their rudeness and immediately said, --
"This walk is not wide enough for our party. We had better go into the avenue."

As for the original question of why he waded into the accomplishments debate, it's possible that he's provoked by the glib self-satisfaction he's hearing. He's less direct than he was in ch6 ("Every savage can dance") and Edmund Bertram is in Mansfield Park ch13 ("If we do not outdo Ecclesford, we do nothing"), but I do like the idea that in the heat of the moment he's sneering behind his hand and trying to send up the whole notion of showy accomplishments. Perhaps it's only later that he realises that a compliment to Elizabeth slipped out. That'd feed into his "Danger? What danger?" moment in ch10.