"Hepburn had been told that her vocals would be used (w/ some assistance on the high notes from dubbing) throughout the film. When she was interviewed at the opening, she apparently still thought that was the case. "
I don't believe she really still thought that at that point, I think she was just trying to save face in interviews. I'm pretty sure she knew by that point that MOST of her singing would be dubbed. But since she probably didn't know the exact amount, and since her understanding was that at least some of her actual singing remained in the completed film (as, in fact, it did in "Just You Wait" and a tiny bit in "I Could Have Danced All Night"), she was able to make a statement about her vocals being "assisted" (or whatever) without actually lying.