anti tank guns miss their shots exceptionally more if there is a green cover portion between the AT gun and the tank.
That does not sound correct at all since vehicles do not benefit from cover. The only difference is at missed shot can collide it item that are between the weapon and target.
this also works the other way around, with exceptions (namely brummbar)
ATG do benefit from cover but only if they are close it so the mechanism is the same.
Unit that fire arched projectiles and unit that fire from higher elevation have reduce change of colliding with shot blocker so Brummbar is nothing special in that regard.