] In my opinion, I interpret his fearlessness as being that Darcy knows exactly what Miss Bingley's reaction will be, knows what her feelings will be, and that his comment is meant to hurt her.

I don't know if he wants to hurt her exactly, but I do agree that he can see that Miss Bingley likes him and he does not want to encourage her hopes. I'm sure he is annoyed by her because she never seems to get the hint, for example when she continually talks to him when he is writing or reading at Netherfield. Perhaps he tried more tactful hints earlier on in their relationship, but they didn't work, and now he has to resort to just being honest and trying to ignore her when he can.