So Great Rift Skyguard.
It has a feature called Axes From the Sky. It calls out the "attacking from high ground" rule to grant the airborne dwarf a +1 to hit because of high ground despite not touching ground and effectively flying/falling/being airborne.
So flying and/or otherwise airborne creatures should intact get a high ground bonus to attack when above their opponent regardless of how or why they are above them, yes?