Bzzt. Silverberg specifically rejected the idea that any woman could write what he saw as such a masculine prose:

"It has been suggested that Tiptree is female, a theory that I find absurd, for there is to me something ineluctably masculine about Tiptree’s writing. I don’t think the novels of Jane Austen could have been written by a man nor the stories of Ernest Hemingway by a woman, and in the same way I believe the author of the James Tiptree stories is male."

He was just plain old wrong.