Submarines can be attacked by air units in this game.Â Of course, they will only get one shot before the sub(s) can withdraw.
You may be thinking of the rule that says subs can’t hit air units, or you may have read something concerning later versions of A&A in which air units are unable to hit subs without a friendly destroyer present.
The underlined portion was a totally new thing for us when we started playing 1942.2 (after playing the classic version for so long). I can see the idea behind it though. Helps preserve the sub a bit better from what we’ve seen.