It’s not a diffractive effect. It’s refraction. Hyper physics has an article on this specifically:

http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/corona2.html#c1

A diffractive effect would exhibit a lot of dispersion. On the other hand, refraction through the flat crystal surfaces responsible for the 22 degree halo does not disperse the light.