It's true that the animation is quite classic, but I think this kind of movie doesn't focus on that but on capturing in the right and authentic way the story of Anne Frank, and well, from what I read in your review they did that quite well.
Thanks for sharing this interesting post with the community!
Let's no forget also, Anne no Nikki is one of a few anime films intended to show what happened in Europe during war. Most of the war anime are focused to WWII but in the Pacific, mostly Japan.