This is not a very meaningful claim since in modern physics momentum is not "mv" or any such simple formula. Momentum is the Noether charge associated with spatial translation symmetry which for field theory typically means the integral over space of some expression involving the fields and their derivatives. In general relativity things are even more complicated. Strictly speaking momentum conservation only holds for spacetime asymptotics which have spatial translation symmetry. There is no good analogue of momentum conservation for e.g. compact space.

Nonetheless, the EmDrive drive still shouldn't work (and probably doesn't work).