Yeah, it's a bit of a retcon, but one that was handled well, albeit clumsily. Darth was probably going to be his real name at one point before they decided it was more appropriate as a title.

I always like to re-interpret that scene as Obi-Wan using a bit of wit in the heat of battle to chastise his former apprentice. Him referring to Vader as simply "Darth" is a refutation of Vader's claim that he is the master/more powerful one, and it drives his point home that he would never be better or more powerful than a Jedi Master because he had already embraced the Sith ideology and even taken their title.