I didn't say "should only teach". If your (obvious facetious, I know that) argument actually followed mine, it would say "A teacher only knows DOS, they would be most effective at teaching DOS". Which is true.

The point, though, of the parent commenter was that just because a teacher can teach something "effectively" doesn't mean that something should be taught in the first place.

There are plenty of teachers throughout the United States who are effective at teaching creationism to their students. I think we can all agree that doing so is not healthy for those students.