Not sure why Cooke would say that...
It may very well be a matter of opinion. They may consider 30mm the acceptable image circle.
I measured to what I considered reasonably usable in regards to light and focus falloff.
They may also want to keep things simple and not list a mere 2mm difference between some lenses and not others.
Just my guess.
Well, then I look forward to somebody doing some tests with the Panchros and 5is on 5K. Hopefully you'll get to test one or both sets in the reasonable future.