Fresnel lenses don't operate on diffraction. They operate on the same concept of refraction as a normal lens. They just remove the mass of glass away from the curved edges that don't provide any additional refraction, and then collapse the edges towards each other.
Diffraction usually refers to interference patterns that may or may not be due to some level of refraction.
I am guessing that if a fresnel lens has a high enough number of fresnel rings, then you may have diffraction patterns as well due to the rings very close edges. Maybe that's what Canon is using/referring to?
Edit: Photo.net article seems to give a different explanation, which I don't fully understant yet.
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