Ian Anderson from Jethro Tull plays a flute. The flute is a heavy, metal instrument.
I think Tull only won that because they'd been overlooked for years when they were churning out classic album after classic album.
But let's be honest, Metallica nowadays are fucking twats so I'm glad they didn't win that award.
Award shows like this are pure pish, you might as well ignore them, they mean nothing.