Softness due to diffraction is dependent upon the actual size of the iris opening, not the f stop which is nothing more than a ratio: focal length / aperture size. A lens with a focal length of 80 mm. will have a smaller f number than a shorter lens given the same sized opening. That's why you can stop long lenses down a lot more before diffraction problems begin to show up.
Good note, but I don't think this statement is correct. Since diffraction is an angular deflection, it is the aperture vs. projection-distance that counts, and therefore, it IS the f/stop that counts. See the graphs attached.