Heavy Metal is less syllables than traditional metal.
and Celtic Frost should have black metal listed because
I wouldn't argue against including it as a secondary genre even if they don't completely fit. Just calling them plain ol' thrash doesn't seem to cover it.
I disagree with the inclusion of alternative metal for Type O Negative. None of their albums really fall into the genre. RYM has only Life Is Killing Me list it as the third genre on there and that's probably their least important album anyway. AMG lists them as alternative metal, but I don't think you can take their genre listings on the sidebar seriously, otherwise you'd have to add "symphonic black metal" to Opeth. They're usually better with genres in their reviews, but even there they tend to fuck up. Their review of Slow Deep and Hard
calls it "goth rock", when it's pretty much doom with a lot of thrash and hardcore influences carried over from Carnivore and while there are a few Gothic influences, they're not as pronounced as they would be on the later albums. Their later albums have a few songs with pop rock sensibilities, and they probably got some airtime on alternative rock radio, but I think doom and Gothic metal should be sufficient.