I can see how a term like that might spring to mind to refer to someone who was saying that some peculiar variation on the music ought not to be allowed, and that anyone who tried to do it ought to be roughed up or something. But just for saying that they don't like it? Or that it didn't fit in their understanding of the term "folk" - seems a bit over the top.

Actually the Nazis were very fond of the word "volk" themselves, and there were plenty of real folk Nazis. Just as in Serbia and Bosnia these last few years there've been folk musicians encouraging genocide, and mass murderers sponsoring folk bands. Real folk Nazis. Just because it's folk doesn't mean it's wholesome, unfortunately.