How does the Sword of The Thunder God get stopped by a Kunai?
Fillers are parts of the series that are designed to give the manga time to get a larger distance from the anime.

Generally, less experienced (and less paid) writers do these fillers, not the original mangaka.

This results in inconsistencies with the general storyline, and sometimes, even inside the episodes themselves (due to the writer's lesser experience).

Now I haven't checked the credits for that specific episode, to see who was the script writer for it, but my educated guess is that this is the case. A plot hole, and nothing more.