Except that you're missing the point where the VLSD locks up as the sheer forces increase temperature. The sheer thickening fluid transfer is what transfers the torque (heat independent), and the heat generated induces lockup. Infact, fluid only serves to provide smooth lockup, it's mostly the plates acting in a clutch-like fashion that do the torque transfer if needed. As subaru states, the hump phenomenon serves to eliminate the rotational difference between the housing and the hub, directly coupling the front and rear axles.
Further discussion on VLSD from a SAE paper and VLSD design evolution, from sheer only torque transfer to sheer-couple torque transfer: