But logically jackill is right. The ISU and the Sturmpanzer IV use the "same" howitzers, why shouldn't they be HE. And the Sturmtiger uses an even bigger howitzer/mortar (with rocket grenades as I just read).
For mor depth they could introduce an option for the assault guns (Stug, SU-76) to switch between HE and AT grenades. Though the SU has already the barrage for that and the Stug the HEAT round, so...
Well, first because PQ said so.
He also said only arty use HE rounds.
Following this logic, mortars and rocket arty also uses HE rounds.
Which leads to the conclusion that shells that do not target entity or vehicles, but ground instead are all HE shells, which mean ISU concrete piercing round, B-4 direct shot[thou this might be an exception] and future sturmtiger rocket shot(because anyone thinking it will have normal autofire and won't be short ranged, mobile B-4 is mad).