Is this plasma-thruster experiment actually measuring thrust?
No, it's not measuring thrust.  It might be measuring a proxy for thrust density (force per unit area).  Anybody could use their setup to measure a ducted fan or a turbojet or even a rocket, but nobody does.  They themselves could have calibrated their rattling-ball instrument with a conventional thruster of known, conventionally measured, thrust (a $20 electric ducted fan from a hobby store).  (What's the threshold for rattling, anyways?)

As Jpe61 commented, it would have been much cheaper and simpler to place their thruster on a load cell (a fancy bathroom scale) and observe how downforce increases when the thruster is running.  $100 instead of fancy metalwork.  See, for example, fig. 6 and reference 23 of Prasad et al, "A laser-microfabricated electrohydrodynamic thruster for centimeter-scale aerial robots," https://arxiv.org/pdf/1906.10210.pdf.  See also the fancier inverted-pendulum thrust stand used at Princeton to measure plasma thrusters.