Tank does not have to be penetrated to be destroyed.
True, the SU/ISU-152 frequently destroyed German tanks with high explosive rounds; even a near-miss could be fatal. The round was powerful enough to flip a tank over.
The effectiveness of AT weapons has to do with energy. AP rounds use kinetic energy to destroy tank while HE explosives use chemical energy.
Also true but, the gun of the ISU-152 fired a high explosive artillery round, not a HEAT round. The damage was almost exclusively from the blast effect. The AT round wasn't used very often because, the low muzzle velocity of the gun meant it wasn't that effective. The HE round was a more reliable way to destroy a tank. The Soviet HEAT round wasn't developed until after the war.
Real life fact have secondary role in the units performance, else the Brumbar should have a similar penetration and damage of ISU-152 since it was 15 cm Caliber gun.
It's true that real life facts have next to nothing to do with this game. However, the Brumbar had a way lower muzzle velocity (790 fps) than the ISU-152 so, even hitting a tank was difficult. It did have a HEAT round but, it would have to get very close to guarantee a direct hit. Like the ISU-152, using HE was more reliable because, even a near miss could be fatal.
My overall point is that it's a waste of time to bring "real-life" facts into discussions about the game because, the game is so far from real life that there's no value in comparing them. In real life, the ISU-152's AT performance was nothing like it is in the game; it did not fire a high-velocity AP round. However, it would be very difficult to model its true behavior in-game without causing balance issues so, we're stuck with what we've got.