I don't think she is an intermediary figure in this novel at all. The problem is with what Defoe does with Barton's story when he receives it. This excellent and incredibly thought-provoking tale forces us to examine the underlying patriarchal assumptions that form the foundation of so much of our society. Suddenly having a female figure that is the hero of the tale, Defoe treats the principal protagonist as an intermediary as he goes through the process of editing her original story to make it more popular with the public.
This process of course involves the complete deletion of any mention of Susan Barton and also any female perspective or traces to the original tale. Susan Barton is not an intermediary in the novel, but she is made into one thanks to what Defoe does with her tale. Coetzee therefore makes a profound comment about the way in which the female perspective and gender is constantly sidelined, if not completely erased, from literature, which is, he suggests, fundamentally a patriarchal institution, edited and selected by men for men.