If it wasn't, why would have have expended so much energy trying to get Comey to disprove its existence?
http://nymag.com/daily/intelligencer/2018/04/im-a-peeliever-and-you-should-be-too.html
https://www.cbsnews.com/news/golden-showers-trump-pee-tape-james-comey-book-interview/