Or at best they are matched and Lukes fury gives him the edge.
Congrats, you figured out the intended goal of the scene. It’s supposed to show how powerful anger can be, showing why people would be tempted by its power. Luke trained himself before the duel, him and Vader are very similar in pure skill. But when Luke gave into anger, it allowed him to win easily. “Quicker, easier, more seductive,” as Yoda said.
I would not say that is the main reason. ROTJ spends a lot of time showing that Vader has an inner conflict (plus the final scene in ESB), and that is the main reason for his poor sword performance in the final duel.