Then the result would be the same if she gave birth on an operating table four feet off the ground, wouldn't it?
Ah, but that wouldn't illustrate the absurdity of the legal theory! That is the entire point! :)
By putting our expectant mother zipping around above the clouds, we see how easy it is for a child to be a citizen of here.... or there..... or somewhere else! That the child's citizenship is so heavily dependent on the arbitrary notion of a defined area and timing is what demonstrates the legal concept to be ridiculous if reduced to it's salient aspect. (birth within a boundary.)
Argumentum ad absurdum. (Or Reductio ad absurdum if you prefer) Proving that something is wrong because it leads to a ridiculous conclusion.