I think this is only a part of the problem. If you check the measurement data sheet, the 300L maxes out at ~50lp/mm, whereas e.g. the 28-135 reaches or exceeds 70lp/mm across the focal length range. The 70-300 non-L also shows higher lp/mm at 300mm. Utter rubbish, even more so than their sensor shenanigans.
Their results are absolutely screwed up. They run counter to the experience of many photographers. A few have testified to that very clearly in this thread.
Is there even a single Canon photographer who would rank the 85/1.8 at the top of all Canon lenses? I doubt that there is even one. If the DxO lens ratings were at all meaningful, then we could all SEE that the 85/1.8 offered the best resolution.
The 85/1.8 is an excellent lens and a great bargain, but there is simply no way that it is the king of the Canon lenses. No way. This result, like so many measurements at DxO, simply doesn't accord with everyday experience.
I find the 85 focal length much more useful than 135, but the 135L is without doubt the better resolving lens. Here is evidence: