is demon slayer: kimetsu no yaiba episode 21 filler?

Seeing Tanjiro try to protect Nezuko even as he collapses triggers memories of Rui’s own past. Once a sickly boy, he became a demon after meeting Muzan Kibutsuji, who gave him a share of his blood.

But after he’d turned into a demon, his own parents try to kill him. Ever since then, he’s been searching for a family.

Seeking a bond that seemed so unattainable…