That sounds pretty OP if it ignores basically every defense stat of a division!
Not every stat.
Divisions with AA guns can inflict damage on planes and disrupt their bombing. The higher the air attack of a division, the more likely it is to do these things.
I've done some tests using older MP saves, and the difference between no AA, some crappy support AA, and significant AA is substantial if the enemy tries to constantly bomb you.
Armored versions of AA guns, particularly heavy SPAA, can murder CAS if you upgrade their guns. And AA guns in a division cuts the air superiority penalty (if you are suffering from it in the first place).
And, of course, CAS can't ground attack if they are getting intercepted and shot down by fighters every day.