Aerial Ace isn't technically an aerial attack. It's based on a sword technique invented by Sasaki Kojiro called the "Turning Swallow Cut", and involves slashing at the foe twice at very high speed, first downward, then immediately up (hence the Japanese name of the move, which translates to "Swallow's Return"). Flying Pokemon generally use their wings for the slashing, if the manga is any indicator, and most everything else just uses claws. Apparently the barbs on Ferrothorn's vine-things count for that.