Of course. It's not unusual to attempt an attack from the air, since it's harder to defend and can take your enemy by surprise. A warning like this might be for someone like me, who can fly under their own power, or for someone attacking from a plane or spacecraft.

[Looks over at her.] Your people haven't developed the technology for flight yet? [The first thing any civilization does with something like that is attack someone, after all, so they probably haven't.]