Zeiss MTF is measured with a real lens on the K8 MTF tester using appropriate thickness sensor cover glass for the target camera platform using white light. Thus: real lens with white light with diffraction accounted for and the actual camera platform. The only other vendor which (maybe) does this (and only for one aperture) is Sigma.
Because of the K8 measurements with a real lens (but no camera), the Zeiss MTF charts cannot account for software correction of chromatic aberration.
See my in-depth discussion of the MTF charts for the Zeiss Batis 18mm f/2.8 Distagon., but here is one point of discussion regarding astigmatism.
As per Zeiss, the astigmatism in the MTF graphs is largely related to lateral chromatic aberration. Not all of it, but a good portion of it. This is a common behavior for ultra wides, including Leica M and other brands.
Correction for chromatic aberration via in-camera JPEG or during raw conversion means that the MTF will be better than shown, namely, reduced astigmatism.
Since Adobe Camera Raw forces-on chromatic aberration correction regardless of camera settings, there will be near zero chromatic aberration and performance will be closer to the sagittal (solid) lines in the graph.