Perhaps I need to explain compressibility error.
At sea level pressure altitude compressibility error is zero, i.e CAS = EAS. In theory this would be true even at Mach .9.
The reason is because the mechanical ASI is calibrated to account for the compressibility of the air at sea level. However the instrument does not know its own altitude. If you use the ASI at a higher altitude (for which it is not calibrated) then it will exhibit compressibility error.
Now a EFIS does know its own altitude. So a EFIS could display CAS or EAS depending on how it is programmed. Airbus and Boeing choose to display CAS.