Its a lot more complicated than that. ion thrust, photon thrust, em thurst, etc are nothing new. But this appears to be.
Imagine a closed plastic box that has a propeller attached on the outside. You put it in water, turn it on, and it moves in a direction. Common, simple, normal physics.
Now fill that box with water, put the propeller on the inside, and turn it on. Does it move in a direction? Of course not.
From what I understand, replace the propeller with microwave generator and for certain shapes of the closed box, the thing appears to move very very little. So this is the controversy, is it really moving or is it just a measurement issue where the micro thrust is within the measuring instrument's margin of error.
If it is really moving... WHY? That is ground breaking and it opens up a new arena in the field of physics.