Sarah specifically said "he" and she is usually quite precise in her choice of words.
“The individual behind this piece has chosen to deceive, rather than support, the duly elected President of the United States,” she said in a statement.
“He is not putting country first, but putting himself and his ego ahead of the will of the American people.”
IMHO that makes it highly likely that she knows the identity of the author.
I agree, if she didn’t know I think she would have said “that person” or something other than “he” because she is very precise.
I thought it was all made up, but after thinking about it I don’t think she would have said all of that without knowing there was a real person and thinking she knows who it is.