The review has a couple of problems relating to a lack of information about which camera he did the tests with and breadth of filters he tested . While he states which Canon camera and lenses he used, in my more limited tests, the different sensor/processor technologies in different makes of cameras make a major difference in color rendering.
Also he doesn't state which Tiffen 10-stop (ND 3.0) filter he tested. Tiffen makes at least three if not four different ones.