I also was startled at the inclusion of mecha in episode 18, but was pleasantly surprised at how well it was pulled off. They used mecha with the same dignity and seriousness that people in the past would have used with sabres and pistols. In the future, we use robots instead of guns and swords for duels. It's normal to them and they treat it as such. And the episode itself, especially the second half, was something special.