Well,. I'm not sure this is true.
For instance,. I "tested" the Sigma 50-500mm for well over a year as my primary wildlife lens.. I love the lens and do recomend it...
But I now have the 100-400mmL and it beats the 50-500mm on every level except zoom range and AF tracking of small moving subjects.
My test was done in a camera shop shooting out the open door at the brick wall and signs on a store across the road. We used a tripod, mirror lock-up and a cable release. Comparing shots at 400 mm it was very close, but the Sigma 50-500 showed slightly more resolution than the Canon 100-400. This is not what the salesman expected, but that was his conclusion as well.
The difference was so slight that it really didn't matter so the real choice came down to whether IS on the Canon was worth more to me than the 500 mm focal length on the Sigma. Since I almost always use a tripod for wildlife and the instructions for the 100-400 said to turn off IS when using a tripod I chose the Sigma.