I think what HamburgerBoy is talking about with that (and I might be wrong), is that some black metal doesn't have much in common with the original ideas behind metal in the 70s and 80s. Like if you listen to something like Transilvanian Hunger or Hvis Lyset Tar Oss, they're all about using repetition of simple riffs or melodies to evoke an ambient like atmosphere. This is something generally associated with other genres of music and not metal at the time. These records inspired thousands of bands, some which took it to further extremes which are the types of bands I think HBB refers to as "prefixed black metal".
Things obviously evolve and change over time so I don't think this invalidates them from being called metal, just because they had a different goal to the pioneers.