I think people are reading too much into a line. “When I left you” doesn’t match very well the Mustafar encounter anyway, since Obi-wan did the leaving. It could easily be interpreted as Anakin leaving the teachings of the Jedi, having failed to gain mastery in their ways, but he’s now a master of the ways of the Sith.
Exactly. And there are already so many other contradictions in Obi-Wan’s ANH lines when you compare them to what is actually depicted in the prequels. I’ve accepted those contradictions, so a few more won’t bother me as long as they are telling a good story.