I said it does not deal 10%, pls read again. Heavy cover mitigates half the incoming damage. The squad might get hurt by the far AoE, only 1-2 models will receive near AoE dmg. Overall it hurts the creo by 10-20% of the squad health
You are underestimating the damage by a big margin. By your calculation you would need 5 to 10 rifle nades to clear a mg crew from cover. This is simply not the case. Even if the squad is somehow super spread out in green cover so you can only hit 1 member per grenade (which doesn't happen in the actual game), it will take 8 grenades max. In the actual game however it just takes 2 grenades and a LMG burst or two.