Not that again! Someone comes up with some reason to say that C is really Functional about every one and a half years.
The author was making a joke about how Haskell uses monads to remain pure.
"cpp" is Haskell, "C" is the way IO monad is used.
This seems to come back to that original Scala-is-not-functional article, which seems to argue that it isn't features or purity that makes a language "functional" or not, it's how good the language is at working with that style.