IMO, a little harsh on the passenger. The lesson is that you need 2 F/As or at least one F/A and one well-briefed passenger in the back in a ditching. Through training or familiarity, crews and frequent fliers know not to use the rear exits. Most people would instinctively try to use the nearest exit, and the safety briefing reinforces that by asking people to note the nearest usable exit. You have to know something about airplanes to know that a rear exit is not usable in a water landing. It may be on the safety card, but in an emergency, the instinct is to get out of the airplane.
I agree that the sentiment is a bit harsh. Having never experienced a ditching I can't really comment on the level of panic in the cabin, but for some pax it was probably an issue. The instinct to get out of a cramped metal tube that is not really seaworthy can lead people to do foolish things, including actions that make it less seaworthy.
It also reinforces the idea that you really do need to look out the window and assess the condition outside the aircraft before opening any door. Its possible (though unlikely) that a ditched Airbus could end up nose down in the water making the rear exits preferable. Granted it would probably mean that watertight integrity in the forward part of the pressurized section of the aircraft had been compromised and the situation would have been a lot worse for all concerned.