I don't think it's a question of the teachers needing to know in order to teach, it's a question of them knowing what's going on in the drumming world and what their students may be exposed to.
It's like me knowing about blast beats. I can't do them, I can't imagine being asked to do them, I don't necessarily listen to music with them... but I know about what they are. And really, that's the extent for me. But, at least I know, and that adds to my knowledge of my chosen career.
Bermuda
I am with you all the way. But what I meant was that no one knows everything about there chosen profession, no matter how long they have been doing it, and learning to teach "everything" would be impossible. You would spend all your time learning new stuff and leave yourself with no time to teach.
If anyone can spend hours every night watching YouTube because there might be something new on there, and there will be, I'll hazard a guess they will be single, and for a good reason.
Do what you do best seems to be a sensible way to go. If, for sake of argument, you are a great Jazz teacher and students seek you out, and your diary is full, why spend time seeking out new developments in death metal drumming just so you can say "Yes I am aware of that"?