The difference is that no other off map arty delivers that amount of firepower in such a short time. Within 3 seconds of the flares dropping, the first tank was already dead. That means that if you were not already watching the tanks and started moving the moment the first flare just entered your screen, you lose the tank. You'd have to have a pretty warped sense of balance to think that is OK.
Yes, and that's all you need to react, doesn't it?
Offmap are all like this back in COH1, and almost all the time before CAS Stuka can strafe, there is a recon pass. That thing is a perfect warning.
CAS can be identified by the ridiculous amount of LMG at the mid game, so be prepare anytime, period.