They aren't, and have never been. Double ATG is so good at fighting off medium tanks that I consider it one of the main reasons players go straight for a heavy instead, now that heavies are viable. It's so easy to make a small mistake and lose a medium to double ATGs, so players choose the reliability of heavy durability instead. The target size buff is meant to make mediums better against ATGs as well as TDs, so messing with the TD accuracy (which would also make them worse against all other vehicles and buildings) is not a solution for the intended goal here.
Yet the changes does not affect all ATG the same:
Zis no accuracy bonuses
Pak 40 no accuracy bonuses
M1 accuracy bonus vet 2
RW no accuracy bonuses
6p accuracy bonus vet 3
In addition it affect the main guns of tank and hand held rockets while the effect on TDs are again unequal:
Su-76 accuracy vet 2
Stug no accuracy
JP accuracy vet 2 vet 4 (vet 5 first strike)
"heavy TDs"
SU-85 vet 2
Panther no accuracy bonuses
FF accuracy vet 3
M36 vet 2
And that means that SU-85/FF/M36/JP gap if effectiveness vs ATG will be greater and their will simply be more reason to build these unit instead of other form of AT.
The goal should be to have less reliance on "heavy TDs" not more.
And my point is that you are actually opening a new wound without fixing the previous one.
Finally if ATGs are the issue lower their accuracy since this change affect many more things other than ATGs.