I believe HTC also dabbles on the concept of "historical representation, rather than technical recreation" - meaning, some factors or issues are altered, left out, or controlled by game play concessions to give out historical situations, rather than try to recreate every historical fact which might bring out the opposite results of making the game ahistoric.
Such examples are flap deployment speeds, combat trims, auto trim sets, etc.
Some things are technically possible but practically impossible - such as, like you pointed out, it was technically possible that some times 20mm shells could penetrate tank armour, but in real life, no one would be crazy enough to drive their plane in such angle and speeds at such low altitudes, to strafe a single tank.
However, in game playing, people do crazy stuff all the time, and go as far as to exploit things which should be pretty much impossible in real life.
So what it comes down to is historical feel over technical facts in some cases. Like the combat trim.
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I'd rather that all tanks should be rendered impervious to machine gun fire, and 20mm and 30mm cannons.
Only the NS-37 37mm(if our Yak-9T carries an AP version of it...), IL-2 23mms, and VickerS should be allowed to hurt tanks.
Is that realistic? No. But the consequences it brings is historic.
People should readily acknowledge the fact that they should up a suitable anti-tank platform, if they wish to strafe tanks dead.
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In turn, change the tank pintle guns, so that when an onboard gunner, or a driver is in the pintle gun position, the tank can be easily disabled/killed by strafing.