Well, he hasn't physically seen her before, but he was determined to. However, he and Fili know her as their personal guardian "Delva", since he and Fili called her that during their years of communicating with her through letters and gifts of nature. That is where their "love" for her, grew. From there, they admired her ever since. 

In the post, she's wondering if he saw, or heard of her as "Maleficent". Or if he knows that she is "Delva". Either is fine.