The fairly brief article was unclear about the copyright aspects of republishing. It would seem that any added annotations would be subject to a new copyright; but any of the original text copied from the new book would still be public domain.

Or am I missing something in German copyright law?

If the purpose of the "annotated version" is to further some "agenda" to prove a point, it might be as well to just leave it alone. If it actually adds to more general understanding of the era in which the book originated, and even perhaps of the author, it probably would be worth doing.

Or am I seeing (possible) conspiracies where I shouldn't?

John