You must be trolling. As a metalhead kid from the 80s who grew up on Priest, Maiden, AC/DC... I can tell you you are 100% wrong. Metallica is metal. They were so metal that when they first came out, it was scary to listen to them, like terrifyingly exciting. And that's a key component of metal. Now you could argue their Black Album is Hard Rock not metal, but that's about it. But Master Of Puppets is a heavy metal masterpiece. Full stop.