This is three episodes later. Has Rei honestly gotten over Mamoru? Hell, no. But she has accepted that he has never been and will never be hers. He has made his choice a long time ago, and he’s making the same choice now. So she backs off and lets Usagi get her happiness. In fact, she demands her to. She reassures Usagi that everything’s okay between them; no hard feelings, and that she has already moved on. I think a part of Usagi knows that Rei’s not completely telling the truth, given how touched she looks, but she appreciates it by not calling her out on it. Because just like Rei wants Usagi to be happy, Usagi also wants the same for Rei.