When I first read the novel (perhaps the first several times) I thought as you do; however, Bingley says, "But there is one of her sisters sitting down just behind you..." which suggests to me she is close enough that Darcy should have taken some care with his words. Perhaps because of the music and the crowd, and because he is facing away from her (he has to turn around to catch her eye) he assumes she could not, but if nothing else it is still thoughtless of him.

I don't think this is a test to see her reaction, because Darcy simply isn't interested enough in anyone there.