Not quite correct.
www.cambridgeincolour.com
In the very link you posted it even explicitly says "Diffraction thus sets a fundamental resolution limit that is independent of the number of megapixels, or the size of the film format. It depends only on the f-number of your lens, and on the wavelength of light being imaged. One can think of it as the smallest theoretical "pixel" of detail in photography."
which supports the claim that diffraction has nothing to do with the sensor or its pixel size.
The diffraction limit of a sensor is merely the f-number above which it is not possible to take full advantage of its high pixel density, but this doesn't imply that the result is any worse than from a sensor with a lower pixel density. Given a particular f-stop there is a particular smallest pixel size whose further decrease will not give you more resolution due to the diffraction limit. But this doesn't mean it will give you *less* resolution. It will simply not increase any further. A sensor with higher pixel density always provides at least as much resolution as a lower pixel density sensor. But, after the diffraction limit has passed, not necessarily any more. And you can always downscale the image to get the (almost) exact same result.