So, at some point in the manga, supposedly no demon slayer has killed an upper moon in 100 years. And these upper moons have killed several Pillars previous to the anime/manga. Later in the manga,
Some Pillars die in the hands of upper moons, some of them help defeat upper moons or kill upper moons with the help of other demon slayers. But Zenitsu very quickly manages to kill an upper moon alone.
Has Zenitsu become stronger than a Pillar?