Although Helen was blind and deaf, that had not always been the case. Until she became very sick with a fever at age two, she could see and hear. Even after she was plunged into darkness, she retained dim memories of sight and sound. In the quote she cites, she means to communicate that the memory of what it was to see and hear could not be totally erased. She had some concept of what things looked like, and what sight and sound were like, and she appreciated the difference that made from being born deaf and blind, which would have made her journey harder. She is acknowledging that she had a tiny bit of an advantage over what could have been.

It's hard for most of us to imagine remembering back as far as she did, but her sensory impressions left their lasting imprint. These fleeting memories helped her later in life, especially after Anne Sullivan came and began to teach her. As she says, right before the quote:

during the first nineteen months of my life I had caught glimpses of broad, green fields, a luminous sky, trees and flowers which the darkness that followed could not wholly blot out.