There is little that justifies this kind of insanity. If tumah is only up, how far up? The stratosphere? The ionosphere? The magnetosphere? Do Kohanim astronauts have to be sure they don’t orbit over a cemetary? What if a Kohein lands on the moon, does it travel there, or does he just have to worry about lunar cemetaries?
I get that driving a car through a cemetary isn’t okay because it’s not a fixed vessel but the airplane is tens of thousands of feet up. That’s miles away. On the ground, that kind of distance might put you outside of city limits, never mind outside cemetary grounds. I don’t buy Kanievsky’s psak at all, it’s stringent for the sake of being stringent. Not being a Kohein, I’ll never have to worry about it personally but it’s just insane.