Just the same, you can argue that many notable black metal bands aren't completely black metal. What's the point?
It's actually amusing how people make such a fuss over Dimmu Borgir not being black metal enough. Yet, there's hundreds of black metal bands considered to be "avant-garde", "experimental", "industrial" black metal or "post-black metal" that are just as far removed from the genre's roots as Dimmu.
And where do you even draw the line? Do you have to be at least 51% black metal? 66.6% black metal? Or do you have to 100% black metal? Is there actually an objective way to calculate how black metal a band really is? This sounds pretty ridiculous doesn't it?