Do we honestly know enough about the Doctor's past to say they might not be past incarnations? He is a Time Lord with a boatload of secrets after all.
He's also a fictional character whose adventures have been related by many different writers and producers with different sets of assumptions about the character and the series' continuity. Every story except this one has agreed that Hartnell was the first incarnation of the Doctor. Even in the Cartmel Masterplan, where the Doctor was a reincarnation of the Other, Hartnell was still the first incarnation of the reborn individual who knows himself as the Doctor. If one story claims something that every other story disagrees with, I don't think we should ignore the preponderance of evidence in favor of a single anomalous data point. Particularly since it's such a minor element of the story, just an odd bit of trivia.