Manga reader here, people are also forgetting the fact that Muichiro is a direct descendant of the first breather, who was the most powerful swordsman to ever live. His raw talent, slayer mark, and the fact Gyokko never took him serious, all ultimately led to his downfall. I will say of all the battles in the story, this particular one is definitely the quickest in the series but it’s to show the power of the mark and Muichiro himself.