Then why did she fight him off as reported in this story?
Resistance, even with nails, is both expected and harmless. DSK probably loves the struggle. She got away only when she risked a more serious damage by pushing the assailant into a sharp corner of the furniture.
Of course every armchair tactician would tell you that you should use plenty of force and as early in the fight as possible - this is easier on everyone. Perhaps a man could resist in such a way - not every man, though, and perhaps not even one in ten men. The ratio of fighters vs. fleers is even lower among women, for reasons that are quite obvious. This maid was in the "flee" mode all along, and she only fought to the extent that she thought was sufficient for her to escape. A fighter would go for a disabling move. A single straight finger would go far, if aimed into the eye; even a small woman can do it, technically. But not psychologically; such a move probably never even occurred to the maid. Besides, she probably had no clue what the police would do to her if she kills the rapist. She would be right about some of that, actually - if she injures DSK but not kills him, she'd be as good as dead. DSK had enough power to issue a contract on her, and would have enough spite to do it.