I'm not going to say that I completely agree with Buckley's statements, but those examples you used probably sound too similar to prove that sub-genres are important, at least to someone less knowledgeable about metal. Your examples of black or death, for example, sounded pretty much the same to me at least. The only real difference was the fact that death metal has growling lyrics and black or thrash metal was more of a regular yelling voice.
I agree that metal is way more diverse than "just screaming and grunting", but there were other examples that would have been better to prove this.