Canon EOS 5D Mark III Review

6.9-7.3 versus 4.88µm for the D800 means diffraction will be noticed at wider apertures, that is a given but still, the finer pitch means that at any aperture, even past the point of diffraction will still produce more detail and data per unit of pitch.
That is physics, the relationship of aperture edges to size of lens are the only factors in determining diffraction. Even if diffraction at an extreme causing the interference patterns equal to the diameter of the detector, an extreme case indeed, it would still resolve just slightly less than the D4 or 5DIII at their theoretically best resolution, with an infinite aperture. But introduce any diffraction inducing apertures to the D4 and 5DIII and even with their lower resolution, with less acuity than theoretical peak resolution will result.
What is so mysterious about this, it is well known over hundreds of years of telescope design and application and is no different with film or digital. Finer grained film even after diffraction was capable of capturing finer detail than courser gain film.

There is a lot of confusion of what diffraction is and isn’t. The fact that a higher resolution detector sees the impact of diffraction before a low resolution detector has little to do with the detector other than it can detect differences in interference patterns created by different lenses and apertures at smaller Airy Disk diameters before a lower resolution detector.
Try this analogy: If you measure a board with a measuring tape marked in cm and measure the same board with a tape in mm, the latter tape will show a great deal more data detail than the former tape. The tape did not change the board or reality, it just allowed finer resolving of the data. The board still looks the same from a given perspective. Add diffraction to the both tapes, say 1/2mm. The latter tape will show more ambiguity than it did before, and will be called Diffraction limited. But it STILL out resolves the cm tape by a wide margin. Diffraction impact on a given optics and detection system is based on the best a system can resolve versus what that same system can resolve after diffraction set in. Comparing to another system means nothing.
So, sure the ultimate resolving power of a D800 is lowered a little by diffraction, it surely does happen but even at high apertures its 4.88µm diameter detector is going to resolve more than a 7µm diameter detector. The courser sensor just means that to see the impact of diffraction it has to be really bad because although it is there regardless of whether a detector can resolve it, the courser detector only notices it with it is more dramatic. But it is also does not notice fine scene detail either. The diffraction is there, but the lower res camera just can’t capture enough fine detail to notice it. If you care about detail, the mm tape is a better tool regardless of system diffraction.
If your understanding of diffraction was correct, we would not need extremely large primary lens telescopes, because they are diffraction limited sooner than a small diameter backyard telescope. Even with diffraction, there is finer resolving power to the larger optical system so luckily, astronomers and physicists do not believe the same as you. Nor do photographers.