is demon slayer: kimetsu no yaiba episode 50 filler?

Muichiro is driven into a corner as he is trapped in water by Gyokko's Blood Demon Art amidst their battle. Meanwhile, Tanjiro and his comrades have successfully severed the necks of the four demons, but seeing that they haven't turned to dust, they realize that they cannot be defeated just by having their necks severed simultaneously.

At that moment, Tanjiro senses the presence of a fifth demon as Genya aims for its neck...