I'm no engineer and I won't attempt to dispute findings for or against the Lego model. I do have (1) a basis for comparison and (2) empirical evidence.
1. When we first began towing our combination (30' SOB with Toyota Sequoia), we did so with a conventional sway control weight distributing hitch. The dealer set it up, maybe correctly and maybe not. We experienced sway.
2. I installed the 3P myself according to the instructions and after much back-and-forth with ProPride to ensure I was understanding them. I believe I installed it correctly. After 20,000 km of towing in all conditions (including a cross country trip with wind, rain, prairies, hills and mountains) we have experienced no sway at all.
By all means, theorise about the design and attempts to explain its effectiveness. I have moved past theory into proof.