In the first Star Wars film (retroactively retitled A New Hope), Vader says to Obi-Wan Kenobi "When I left you, I was but the learner. Now I am the master."
Yet, Vader and Kenobi duke it out in episode 3 of Obi-Wan Kenobi ten years after their duel on Mustafar (during which Obi-Wan made schnitzel out of Anakin). During their encounter in Obi-Wan Kenobi, Vader has the upper hand throughout, and Kenobi only escapes through someone else's intervention. This occurs chronologically seven or eight years before the original Star Wars film.
Is this a retcon of the original Star Wars movie, since Vader clearly masters Kenobi almost a decade before episode IV?