There is a long history of discussion on this. Theoretically, MOI is the sum of (mass*distance^2) for all mass points at various distances from the axis of rotation, and since distance^2 is always positive, there is no way mass can be added anywhere to lower swingweight.
However, I have measured lower swingweight when weight is added to the handle (on a Prince machine).
I think it is due to how the instrument works.
From the player's point of view, the added mass in the handle makes the frame more head-light and can give the impression of swinging easier.