I don't recall if it was an Airbus placard or not, but I have certainly seen pictures on an emergency placard of exit slides being used as a raft during a water landing.

One reason that passengers had to stand on the wing instead of being in one of the rafts is because the tail section immediately submerged, so the two tail exits were never opened and those two slides were never deployed. The cabin crew directed the passengers away from the tail exits.

With several boats close by, I also wonder if the crew was calculating the danger of overloading the rafts by piling everyone on the rafts versus waiting on the wing in anticipation of imminent pick up?