If one wishes to get technical about semantics, DI can be applied to aerial maneuvering. The logic that follows is rather simple, really. DI stands for "directional influence," which is just what it says, that is, influencing your direction. Under most circumstances, however, Smashers generally only talk about DI when dealing with getting hit by an attack. In this instance, DI is used to
alter your normal path from the hit received. Likewise, this same line of thought can be applied to maneuvering while in the air. If falling straight down is the "normal path" while falling, then moving left or right while in the air
alters this normal path; the similarity to DI'ing while actually receiving a hit is quite clear.
Again, this is just being technical and exacting.
Also, DI'ing is for all intents and purposes the act of dictating (to some extent) the path taken by your character. Therefore, maneuvering while in the air likewise dictates your aerial path and thus is an example of DI, albeit an example outside of the norm of DI'ing during a hit.