Granted Darcy is intelligent and a man of consequence and he wants her.

Lady Catherine de Bourgh is intelligent (perhaps) and a lady of consequence, but she couldn't get Elizabeth to stay longer at Hunsford.

Elizabeth did not seem to be intimidated by consequence or intelligence.

I suspect surprise at suddenly realizing she might had been wrong about what he felt about her was part of it, because she was sure he disliked her. They had been trading somewhat tense conversation since the Netherfield ball (the dance)(also when Jane was ill at Netherfield). The other part is that she literally unloaded a backlog of emotion on him. That has to be draining. As you say, Darcy was a worthy opponent, although I'm not sure opponent is the word I would use. A challenging peer might be better, even though not in consequence.

Mr. Collins was not capable of engaging Elizabeth on anything like a peer to peer level. He was in his own world and ignored what she thought.

Still, if Elizabeth really disliked Darcy, I could see her writting him off just like she did Mr. Collins.