Try not to think about realism too much.
When I first bought the game, my friends were immediately complaining about the rules.
I tried to justify OOB rules.
But it soon gave me headaches.
A single machine gun might down a single air unit if lucky,
At the strategic level, the probability is approximately zero.
Even powerful fixed AA placements had a hard time hitting air units, you thing mobile AAs in infantry or tank divisions can do anything? Let alone the little guns on submarine deck.
Destroyer guns had a hard time penetrating battleship deck.
You thing guns on auxiliary ships (transport piece) can do anything?
Now, lets hope we haven’t confused the OP.
Infantry kills fighters yes.