Implying you couldn't figure out by meeting them. I'm guessing just like people who listen to Christian music these people are loud and proud. Probably have SS tattoos on their neck. Unless it's actually decent music and random listeners just got doxxed as Nazis.
It's quite simple; they're talking about the fact that people who listen to both kinds of music tend to wear their beliefs on their sleeves. This is not a complicated idea.
Ok so when you compare things it doesn't mean that you think they're identical, for example if i compare the similarities between the number one with the number two I'm not saying that 1=2; they are both sigle digit integer values and someone can say that without equating them.
The statement made was that Christian music fans and nazi music fans share a trait, though the trait is expressed differently - that trait being that they are very easy to identify from outward appearances and that it would be hard to know one without knowing about their beliefs. People who listen to Christian music tend to wear crosses and talk about Jesus a lot, Nazis get swastikas tattooed and talk about Hitler.
That doesn't mean they're the same in every regard or morally equivalent, I have a lot of things in common with Hitler and so do you, if someone pointed out the fact we all have respiratory systems that doesn't mean they think we're culpable for genocidal war crimes.