Although I know my answer has been already accepted and I still find the sentence in question to be very odd in its own context, I have been reminded that が is not totally unacceptable.
As part of my attempt to understand what is going on, I have come up with a scenario where 〜が田中さんです doesn’t necessarily sound weird to people who have never heard of him.
Suppose someone is introducing a sports team or something, and that person says:
This wouldn’t sound particularly odd to me even if I hadn’t heard of Tanaka-san before. I would only expect something like 副キャプテンが山下さんです to follow.
I think キャプテン here can be seen as one of a number of aspects of the topic, which in this case is the team Tanaka-san is the captain of, and therefore, the above sentence can be understood as describing something (チーム) focusing on a particular aspect of it (キャプテン). The particle が may be used to exclude other aspects (e.g. 副キャプテン) for better focus. I cannot explain why, but I feel the aspect to be focused with が has to be picked from a limited set, rather than an open-ended one.
So, if you can see ベルリンという名称が歴史的文書で最も古く遡れるの as the first of a number of aspects of Berlin’s history to be discussed in that context, the sentence in question might not sound too odd to you. This is difficult for me because I cannot find anything that would correspond to 副キャプテン in the subsequent part of the history section.