I disagree with that analysis.
When the image is larger then the Airy disk it is no longer diffraction limited by definition. So when one has exactly 1/4 wave optics they produce an image in which all the light just fits inside the Airy disk. So this is the upper bounds of the shaded yellow area. So one can relate the theoretical diameter of the spot formed when using a Paracorr which in this case is labeled Airy disk radius to the wave front and estimate the amount of spherical aberration the Paracorr is generating.
1/4 wave of SA in the wavefront modifies the MTF and changes the image in a detectable way in the star test and on the test bench.
The Airy disk is determined by the aperture and f/ratio of the scope, and would be no different if the scope had ZERO spherical aberration.
The idea that 1/4 wave is somehow "diffraction-limited" or essentially produces a perfect image no one can tell apart from perfection is a historical, experiential, analysis by 18th and 19th century observers. 1/4 wave SA is very noticeably different on an MTF graph from perfect optics.
So your assumption the Airy disc size represents 1/4 wave of SA is incorrect. It represents theoretical perfection. The wavefront error that produces visible errors in the star image can be much smaller than 1/4 wave, per Suiter, and Aberrator, though the aberrations that throw light out of the Airy disk into the diffraction rings will reduce the size of the disk rather than increase it.
Though, I agree, if the SA added by the coma corrector is additive to that already present in the optics, it's not ideal. But it is obvious from the graphs the addition will be very small. I can vouch for that, since I DO know how to perform a star test (and I usually see all kinds of aberrations in scopes, the two most common of which are astigmatism and spherical aberration), and the With/Without tests using the Paracorr II display no visible axial changes at all in my f/5 scope.
Of course I cannot extrapolate those results to f/3, but I can speak about f/3.6 and a Paracorr I, where the improvement in star images was profound. And that aperture was always seeing limited anyway.
There are many things to be concerned with in optics, but the added SA from a Paracorr isn't one of them.
But don't take my word for it--call up TeleVue and ask.