I agree with Atlantis and Petra. If the letter is genuine and has not been redacted to protect the original source, it would have the identifying marks they mentioned.
They are wise in not releasing their copy of the letter prematurely. The Watchtower would like nothing more than to say, "Look! Those mentally diseased apostates are spreading lies and fake news again! You can't trust anything they say!" Heck, I wouldn't put it past the Borg to plant such a letter themselves, hoping it would be mistaken for authentic and publicized, just so they can discredit their sworn enemies.
My personal opinion is that this letter is not authentic. It's not hard to write in the style of the Watchtower and make something sound like it's coming from them, especially if you get your grammar, punctuation and spelling right. Part of a nefarious scheme or not, I think this letter is an example of "fake news." I won't believe it or share it with others until Atlantis and Petra verify its authenticity. I trust them.