No it doesn't, it says in the qouted example that the walls were weakened with catapults first. That doesn't mean that it was necessary to weaken the walls first in ever single case of sapping. And yes everybody knows that it's extremely difficult, and if anything it was a last resort. But the point is that for a faction that didn't really have any other alternative options like barbars, they would have done this when needing to bring down walls.