He didn't actually say it in his book. It was a joke that turned into a widespread rumor but it doesn't mention fucking a couch a single time in the book...
That being said... There no way to prove that he DIDN'T fuck the couch. He just didn't write about it in his book.
The funniest thing to me is that they think we actually believe it to be true. These memes are hilarious because he just feels like a couch fucker. Regardless of whether or not he has fucked a couch he definitely gives off the vibe.