I completely agree that there is no conclusive evidence. However, it is the number one activity that influences cognitive performance in all animals that have been tested. Saying that sleep is not proven to be important for intelligence because the causal link has not been established seems a bit like saying exercise has nothing to do with muscle growth, because the full causal chain has also not been established (we do know a pretty full causal chain in this case, so maybe it is not the best example). I think if you were a betting man, and as a scientist you have to be to some degree, you would put your money on sleep supporting some essential process for intelligence.