He is mostly correct. The kill radius only aplies the the barrage unless i am wrong. Wich is a activated ability not its main gun.
The su76 is underwelming esp at vet 0 as a td for countering medium armour and up. The barrage is nice but is a secondary role and less powerfull then the zis barrage.
So yes it is a benifit but not by much imo.
The OHK radius is increased by increasing the damage. It technically also applies for the normal gun, but for AT specialists this is negligible. The SU76 barrage at vet2 gets an OHK radius of 0,96 compared to 0 at vet0.
For the stats and cost, the SU76 is actually decent. In my opinion it is the same issue as discussed in other threads before: For its T3 timing, it does not fit the faction. Soviets need an AI specialist to counter their infantry weakness in the mid game. In the late game, they just have better options than the SU76.
The only point where I can see it used is therefore a last ditch effort to not be overrun if you lag behind in the game or potentially an Ostheer player spamming T3 units. But the last one you will only encounter after already having invested into T4, so you'll usually stick to T4 units as well.