It also has less scatter than the ostheer mortar and brit mortar pit.
Did some screwing around in cheatmode and even though you're right and frontal hits don't pen that reliably, they still often do (it has something like a 45-48% chance to frontally pen a sherman right?) a lot of shots inflict their splash damage on it and some will hit the rear, especially when they get fired at an extremely high angle for whatever reason. In effect, they will deal more damage to tanks than the penetration values suggest. Not that this is empirical testing in any way, and not that this makes leigs stunningly OP, but it happens. They also actually hit vehicles a lot more than the ost mortar it seems.
The very scientific replay.
This reminds me a lot about how people were complaining about OKW veterancy making them op, then a while later everyone realized veterancy is not even working at all.
Yes, the ISG can damage tanks through penetration. It does, however, not have any deflection damage. Meaning it has to penetrate, unlike all other mortars who will do their damage regardless of penetration or not.
In any case, the Leig as an artillery piece is not even close to being good.