AA Artillery is a land unit, part of the Artillery class (12.1).
However, it’s worth noting that fortifications are placed across a border and protect from attacks across that border (Rule 12.7), except for some Special Fortification Zones where the fortification protects the entire zone (12.8). The Maginot Line also has its own rules (1.23).
Therefore, if you can get your planes to attack from an unfortified border, the fortification bonus for the AA would not apply.
The same is true for the attacking land units if they can flank the fortified border.