is demon slayer: kimetsu no yaiba episode 56 filler?

The Hashira, Sanemi and Iguro, along with their comrades head to an abandoned castle on top of a mountain for a mission, while Tanjiro is being treated at the Butterfly Mansion for the wounds he suffered at the Swordsmith Village. Later on, the Hashira gather at the Ubuyashiki Mansion to hold a Hashira meeting, where Muichiro reveals how the marks manifested.

In an effort to strengthen the entire Demon Slayer Corps from the bottom up, the Hashira Training begins...