The law that you linked says that drivers aren't allowed to drive at an unreasonably slow speed, and defines that to be (for highways, with good weather, and just on the leftmost lane) 80 km/h.
It doesn't say that the speed limit is a target, or that you're supposed to drive at or near it.
Yes, but it's how we're taught at driving school. I think it's written down somewhere but can't find it at the moment, so the article I linked to is the closest I could find.
It doesn't matter. People have all kinds of things they are taught about driving from their parents, instructors, and even official documents that don't carry legal weight (e.g., a highway patrol website).
Even when these types of ideas are good ideas, they aren't binding and you can't count on others to follow those rules. The only true rules of the road are the subset of the laws that are enforced; where enforcement might be done by law enforcement officers, civil judges, or insurance company adjudication processes.
It doesn't say that the speed limit is a target, or that you're supposed to drive at or near it.