How can metal be prog?

I've always known prog to be rock music played in a jazz/classical format. Eg. mainly jazz/classical melodies with jazz like guitar/drums/bass and classical keys. It's melody based music with a very even contribution of jazz/classical based instruments such as the usual guitar, bass and drums plus plenty of wind, brass and keys. The guitar is quite often background guitar chipping in within a complex melody or rythm and the drums have plenty of cymbal and snare work, while the bass is busy and really has alot of control of the melody. This music is interesting for many listens because of the melodies and the instruments along with the complexity. This was named PROG by the critics. But metal is the complete opposite. guitar dominated music, usually rythm and riff based where the guitar is up front and louder, the bass tends to just hold the rythm together rather than control it and the drums are mainly based on the toms and keeps the rythm interesting enough until you get bored of it after 2 listens , while there is almost a complete lack of keys. This is metal, but someone called it prog because it's being inventive. But i never thought about prog as being inventive, it's just exciting and it seemed inventive in 1970 because it wasn't standard hard rock