Part of the in-universe rationalization of the use of Nazis depended on Blackwolf finding archival film. There were plenty of other evil groups in history, but for it to work in the context of the film, it had to be a group that existed between the invention of motion pictures and the Apocalypse.
From a production stand-point, archival footage was used, and then image-processed and rotoscoped, to save money. Although other choices might have been feasible too, this points to use of Nazis again, since it narrows it down to real-world footage that must exist, basically between the 1930s and 1970s.
In that time frame there's really only one choice that would come to mind, that everyone watching the film is going to know about, that moreover was well-known for its use of propaganda and psychological warfare (e.g. the Stuka dive-bomber wail), the plot point being after all that the archival film is being used as a propaganda tool.
Part of the point here is that, although real-world humans haven't discovered fairy magic, we can pat ourselves on the back for having excelled in the evil department, so much so that wizards of the future will look to 20th century mankind for lessons in how to be evil. It's a pretty direct criticism of human nature. Again, that makes it work better with the use of actual Nazis.
Just my take.
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