Longerich maintains this case has important censorship implications. “If you accept that a private person controls the rights to Goebbels’ diaries, then – theoretically – you give this person the right to control research,” he said.
A private person controls the rights to Goebbels' diaries until a court of law declares otherwise or they fall into the public domain for some other reason. Courts should have done this in the aftermath of WWII, but Germans wanted these copyrights to remain valid in order to control such writings.
The drive for essentially infinite copyrights comes mainly from the Walt Disney Corporations and the rest of the US Media. Germany has perfectly effective legal sanctions in place to prohibit the distribution of Nazi propaganda - personally I think they're misguided, but they certainly doesn't rely on copyright law.
Arguing as if "research" should be exempted from the usual rule of law is particularly embarrassing for a German professor studying the Holocaust, since many atrocities were committed in the Third Reich because German academics considered themselves above the law and got away with it.
a) research isn't affected by copyright in the same way as publication
b) The Third Reich was, on the whole, scrupulously legal. Once you have absolute power, passing laws to make your atrocities legal is trivial. Which is why the Nuremberg trials didn't apply German laws then, and why we need strong international enforcement of human rights laws today, regardless of national laws.