One thing that I don't think has been mentioned here is that this goes WAY beyond metal. You can look at virtually ANY genre of music and you'll find people who take it to the point of becoming a way of life. And often times, unfortunately, those kinds of people will look down on others who don't take it as seriously as they do. People are people though, and cool people are still cool people... assholes still assholes. I've known a lot of bmers in my day who were as "true" as it gets... yet they still respected me and my own musical choices. While others were just seriously not cool, and thought that anyone who wasn't like they are was beneath them. The latter I simply ignore as best I can. It's the nature of our species I guess. Such as it is.
Black metal was "born" of a very singular vision, so I think a lot of the whole "true" stuff stems from that. But since its creation, black metal has grown into something as boundless as the universe itself... and if some choose not to accept that, then that's certainly their prerogative.
If you ask me, the only "trueness" that matters is whether or not you're true to yourself.