I guess it is not being compared to 1D4
If you look closer at the chart and click the down arrow beside the box that says "5D III", you will see a list of all canon camera models to compare the 1Dx to. The test refered to in the link is not a lens test but a test of sharpness/IQ using a standard lens (Canon 200mm f2) with the only variable in the test being the camera body.
The comparison of the 1Dx and the 5d III indicates(to my eyes) that the 5d III produces slightly sharper images with the standard test lens and the reviewer states that in his review. He even states that the 1Ds III and the 1D IV are comparable, with the 1D IV edging out the 1Ds III. BTW, the 1D IV was released at the same time as the 7D. The review also went on to state that the ISO performance of the 1Dx is better than the 5D III, 1D IV and 1Ds III. Maybe 2/3 to 1 stop better.
When the 1Dx was announced last fall, I was ready to buy. When the 5D III appeared, I questioned my decision to get the 1Dx and asked myself, "why wasn't the 22mp sensor in the 1Dx"? When I saw these tests and read other tests in the past few days from real world photographers, I've decided to adopt a "wait and see" approach. I'm not doubting that the 1Dx is a superb and capable camera, but I'm not convinced, right now, that it is $3000 more of a camera than the 5D III, Sure the 1Dx has a better metering system, faster fps, more customization, ID body, etc, but I'm not completely convinced to pay the extra $3k at this time. I'm sure I'll get lots of "what are you talking about" responses to my statement.
I think what I'm going to do is buy the 5D III and use it for a year and wait for the dust to settle on the 1Dx and see what else Canon may announce in the 1-Series FF body. The 5D III will compliment my new 1D IV nicely and my 7D. In a year, I can sell the 5D III(or the 1D IV/7D), and get the 1Dx or whatever Canon may announce.