You're not wrong. But in your case, you have already assumed that the woman, in reference, is already beautiful. She becomes more beautiful when she believes that she is beautiful. 'A woman' is just a general reference to all women, and I for one agree with your assumption.
But the quote you have shared here is from another perspective, when the speaker assumes that the absolute beauty of a woman comes from her realization that she is beautiful. Whereas you have seen the quote from the perspective of relative beauty. I'm not sure how to explain this. The quote could also mean:
"The real beauty of a woman lies hidden within her, until she realizes that is beautiful."
This doesn't mean that the woman is not beautiful to start with. It's the absolute beauty that the writer is talking about. She may or may not be beautiful in reality, but when she realizes that she is beautiful, any woman will appear beautiful to another person.
This a very beautiful quote. Oh wait ! Haha.