I had heard the same origin story, and you're right about the term changing over time now I think about it.
To pick up @hiram.k.hackenbacker's point, I think the reason people want to subcategorise music, films, etc., is because we naturally want to understand what we like and what we don't, and often those distinctions are very fine. Without analysing every song or piece of muisc individually, we want and need a way to group things together. If those who like "metal" can hear generally applicable trends and differences between songs they have categorised as "thrash" and "speed", and it improves their communication about the music they like, good for them. We don't need to join in, and equally we don't need to be dismissive. Every hobby has it's subdivisions.