The only reason to assume that the III should be used with high compliance cartridges is its very low mass@lewm7 au contraire - compliance matching is dictated by resonance. Mass is just one factor of a resonance equation. "Flexi" is another factor (actually several factors, but let's lump them together). Think of it this way: if you have a wooden pole 12 feet long that bends and flexes slightly (not necessarily the same in each axis), will adding more weight (not mass) at one or both ends make the pole flex less or more?
The correct answer is more. Weight added at then fulcrum increases stability while weight added outside of the fulcrum decreases stability; increasingly so the further away from the fulcrum you add the weight. If you add weight while adding stiffness, or increasing the mass (in this case, weight added proportionally to all the lever) then "flexi" or compliance will reduce.
Who says that science and audio don't have anything to do with each other?