It doesn’t matter about that, it’s different characters on different planes of existence where time can flow differently. And it’s characteristic of Gothic literature for the past to bleed into the present in any case. It was in trying to have a concrete history that the 2nd edition Ravenloft boxed set completely failed to get Gothic literature.
So? No rule says the domain lord has to be a significant character. The focus on killing domain lords is another thing the 2nd edition boxed set screwed up (and why this adventure is so much better than the subsequent Ravenloft adventures - it gets Gothic). The psychic presence of Strahd, and whoever they happen to be possessing, works well as the main antagonist.