As an aside, this feels very familiar as a sci-fi weapon, does that ring a bell for anyone? I was thinking William Gibson first but I can't place it there.

This could be the exemplar prototype for Dune knives that can penetrate a Holtzman Shield. Also recall in Dune there is an aversion to technology, so the simplicity and required human operation is a perfect fit.

Dune's vibrating shield and corresponding weapons.

Raven in Snowcrash? Not quite ultrasonic iirc but extremely thin, like the wire developed in three body problem

It's seemingly quasi-semi-related to The Foundation's 'atomic knives'?

dune?