Boot up the cheat mod and test it. It doesn't counter T-70s like a P4 does at all.
It even loses sometimes.
With the Ostwinds very poor mid and long range accuracy, combined with that penetration (35 pen means only 50% chance to pen T-70 at far range) and its 40 damage means it takes ages to kill a light tank in the average engagement.
I used to play a lot of ost before the ostwind buff and the ostwind always killed the T-70 with ease. I can confirm that I just checked in cheat commands mod and it does even better than it always did right now. If you have problems with penetrating frontal armour you need to close the distance. And if you manage to shoot the rear armour of the light tank, you will kill it faster and more reliably than a p4 would. The strenght of ostwind in that matchup is speed and ability to fire without stopping.
Btw. There is no way you would ever lose a full hp ostwind to T-70, unless you won't realise that it is hidden in your minimal range and let it shoot for a looong time like that.