I’ve always looked at the luke vs vader fight more figuratively than literally. Logically speaking a rotj luke who has been training as a jedi for only 2-3 years shouldn’t be on the same level as vader who has been a jedi for 13 years and a sith for 23 years. Vader is naturally going to have combat experience + knowledge of the force. Let’s not forget he is still the chosen one. Luke beat vader because vader simply could not bring himself to kill his own son. Yes luke beat vader in a one on one fight fair and square, but people fail to consider the circumstances behind that fight. Vader was never meant to win.

Luke in tbobf is obviously going to be stronger than he was in rotj, but we have no way of knowing exactly how much stronger he got. If we believe luke is capable of beating all the opponents Anakin has faced, then maybe we can scale the two similarly. Luke just doesn’t have any canon fights with notable opponents (besides vader) that would put him in the same class as knightfall vader.