Equivalent lenses and diffraction limits

Luminous-landscape has a very thorough article about equivalent focal length and how diffraction limits resolution at low aperture. The fun part is that it's written by two very knowledgeable people and yet they apparently can't agree on the minimum aperture at which resolution becomes diffraction-limited for a given sensor.

Who's right ? I have no idea. I just know that when I use a good tripod and a good lens opened-up to oh-I-don't-know,-about-f/8.0, the result is generally sharp enough.