The thruster seems to be based on ideas of Prof. Mike McCulloch at Plymouth University [0]. The page includes links to a number of publications. His theory seems to be called "Quantised Inertia" (arXiv search: [1]).
I think (not because I have any knowledge about this but because that's way more likely than not) that this is going to be another case where the force they measure is generated by normal interaction with other parts of the environment and equipment. It's great that they are actually testing it in space though just on the off chance that it actually works. Having a thruster design that only requires energy but no working mass would be tremendous progress for space flight.
Good luck to them!
[0] https://www.plymouth.ac.uk/staff/mike-mcculloch
[1] https://arxiv.org/search/?query=Quantised+Inertia&searchtype...
