It is mind boggling that despite the name, they did not (given the (I assume) joke with the name) say, "Ceci n'est pas une pipe" anywhere in the paper. Only once does the phrase, "This is not" occur, but completely out of context of Magritte's most famous painting.
I'm serious - this isn't Joke 101, but this is Marketing 101 in some respects.
I assume they felt it was a bit too on the nose? Anyone familiar with Magritte will be familiar with the painting; any attempts to shoehorn the quote in there would have felt forced, I think. Maybe if they had a website to go with it... but this is just a paper.
Plus, as jpfed says below: The twist here is that this language doesn't use actual pipes; it just creates representations of pipes that can be used by a different interpreter.
I'm serious - this isn't Joke 101, but this is Marketing 101 in some respects.