I said that I disagree with the change proposed, I did not say anything about disabling it.
Because if suddenly there's damage reduction for shooting in the FOW for one unit, every other unit should have something similar too. Otherwise, there's a weird rule that only applies to one unit. And as you said, it'd be a buff to defensive buildings; the change would be similar to a straight HP buff ...
The suggestion had nothing to do with FOW. According to Vipper, ATGs should do less structural damage in general.
That way I also would not say it were similar to an HP buff, since the whole point of this is to make mortars better relative to the ATG (which would also affect vehicles and basically all other weapons).
Still it would take out the most "convenient" way to deal with them. Not a bad thing by default but this potentially could make rebalances for all buildings necessary, since a bunker or fighting position should not force me to build a mortar at 1,5-2x the price. Obviously it all depends on how large the damage reduction of ATGs is supposed to be (10%? 50%?). If it were to harsh then people could just camp with a bunker and ATG behind it. Especially Brits without mortar option would be screwed over, but basically all faction would have trouble. I think though that mortars should first be made better vs infantry targets: static MGs, ATGs and infantry in cover. Currently the meta works completely without structures, so unless we want to bring them back, we need to balance mortars vs infantry and not vs structures.