Right, but this isn't a WWII simulator. In real life an anti-tank gun out in the open certainly could not survive multiple shots from a tank like 30 yards away, but that's something you see all the time in the game.
The main thing that differs RTS from a simulator is the scale, I mean all the soldiers, tanks, guns and so on have ridicolously small ranges in order to make games more dynamic and fit all the units (including artilery and mortars) on really tiny maps.
Of course that doesn't have to include changing units efficiency, field/at guns are low so targeting infantry that close is very hard job even with HE shells (because of the angle) so those could have the same AOE as a tank but much worse accurancy vs low and small targets like infantry.
On the other hand they should still be effective against buildings, that's really important because a gun firing at a building for 15 minutes doing nothing doesn't even look unrealistic but simply silly.