Sonshine, I agree that some people will not believe regardless of what is said. If Johann makes a statement one way or another, some will not beleive it.
Johann is rightly thought to be very close to Linda. It is that close relationship that he has with Linda that causes soome people to beleive that when he says he was asked to post, it was Linda who asked him to post. From that perspective, he is the only one who can clairfy his statement that he was asked to post the letter. He, as he wishes can identify the person who asked him. If he wishes he can tell us that Linda had nothing to do with the posting. She did not ask him to post. She did not provide a copy of the letter.
You may be 100% correct in your statement that Linda had nothing to do with it. But, Johann, due to his statement is the only person who can clairfy what he said.
His continued silence is not serving Linda well. His continued silence contributes to the perception that Linda was involved.