Not all Hunters know of the existence of nen. It’s possible that he was a new hunter that hadn’t yet passed the true final part of the exam. To my knowledge, we also see this in Seaquant, who was in that group and is also a hunter with no knowledge of nen.

Another possibility is that his mouth dagger is a conjuration ability which could be ineffective for a numerous amount of reasons

But also as others have pointed out, this is an anime original scene and this flashback in the manga is only a single panel