I think it depends on what you mean by "historically". My understanding is that the server-side was largely in go for a long time now, but you're also right that before that, it was Python.

While you're right that it's a small and specific part, it's also the core of the whole thing, so it's a very significant part. Dropbox isn't moving away from Go generally any time soon, as far as I know: it's still very much the default.