Received accuracy bonuses from veterancy do benefit infantry VS tank cannons, if only minorly. It makes it less likely that the cannon is able to make a direct hit vs an infantry model.
This isnt a particularly huge benefit, but it does increase survivability somewhat.
Minorly is an understatement, you're talking about markin of 1-2%, 5% if you actually put infantry right in front of tank.
Main gun accuracy stems from scatter, not accuracy, therefore rec acc vet does absolutely nothing.
Grens vet3 rec dmg have larger impact, but since meds do 160dmg, that also is almost completely negated.
Your tests were nothing but scatter RNG.
Against non accuracy weapons that do more then 20 dmg, you aren't going to get any reliable results unless you repeat tests hundreds of times and once you do, you'll come to the conclusion that any 5 man inf is exactly as durable as any other 5 man inf against tanks main gun if MGs are not involved.