From my understanding of the article: the problem is that magnetic field lines are usually spherical. This causes the ejected ions to circle back into the thruster.

[Edit] went and re-read the article

They're using sound or ultrasound to shape or modify the magnetic field. This allows them to eject the particles away from the thruster more reliably.

I'm not sure how that's supposed to work in space but maybe I don't totally understand what a magneto-sonic field is