Lots of good points here. Need to be careful of terminology though. Any lens is always diffraction limited at its maximum aperture, since the Airy disk size is only a function of the aperture diameter. The best performance may be found at smaller than maximum apertures due to better optical correction by stopping down.
The Airy disk size may depend on aperture size only, but aberrations do not.
In most cases, you can't reach the resolution limit diffraction alone would set, because the effect of aberrations is far worse when a lens is set to its maximum aperture.
So almost
no lens is diffraction limited at its maximum aperture.
Stop a lens down, i.e. reduce the aperture size, and the effect of aberrations may (and usually will) reduce. But so will also the resolution diffraction alone would limit it to.
The trick in stopping down vis-a-vis resolution is to stop down (i.e. reduce the aperture size) far enough to limit the effects of aberrations on resolution to the point where the result is equal to what diffraction will limit resolution to at that aperure size.
Stop down further, reduce the aperture size more, and diffraction will take over as resolution limiting factor.
So "Any lens is always diffraction limited at its
minimum aperture".
We must be careful of terminology.
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