I had the same impression, but you explained it better. I also thought Darcy must have realised Elizabeth must be well read, despite her declaration that she is not a great reader. I can't remember if this is after or before their walk, but Elizabeth makes use of many literary allusions in her conversation - such as the Twelth Night bit on poetry and writings on the picturesque. I imagine that in some of his standing in on her conversations, he must have picked up on more of these?