He doesn't seem to have much trouble with Zeiss lenses. And, I am not saying you didn't, but for example if you compare the charts of the Tamron 24-70 f/2.8 to the Canon, you'll notice that in the center at f/2.8, the Tamron is actually sharper than the Canon. The problem I see on those charts is that all lenses suffer more on the outsides. The evidence condemns many Canon lenses as well. So I don't get the impression that he seems to care who made each lens. One more point, he does not only test Canon gear. There are plenty of Nikon lens tests on that site.