At long range, as you say, a heavy breastplate might have stopped a musket round, but you'd have been very unlucky to have been hit at anything over 100m (or even 50m!).
@schizoidboy, it wasn't Bob Anderson, it was the great William Hobbs who choreographed the fight scenes in this movie. As others have said, it's pretty obvious that when the scene begins the duel has been going on for some time and that both men have been wounded and are verging on exhaustion.
Brilliantly choreographed duel between two expert sabre fencers. However, there's a major continuity blooper in there. When Kmicic (Daniel Olbrychski) comes out of the house he's wearing a sabre with a knuckle-bow, but when he's preparing for the duel his henchman hands him one with a crossguard ...
@StrangerYouKnow (and anyone else interested)
Alessandro Barbero's 'The Battle' is arguably the best account of Waterloo in print, completely without national bias.