Thanks - wait a moment, didn't they send those stupid knights to take Leana back ( officially at least)? Yet now she sais they can't get back ( and didn't those two beat up knights that follow them make it back or am I confusing the timeline here?)
Technically yes, but there was a point made earlier where people bored holes in the flying cities below 200 meters where they could safely cross. So there's probably some specific point in the sky where you have to enter from so you don't touch the inner, more deadly layer of the black sea.
There could also be some fundamental difference in their bodies (like the gravity pulling them back) that makes it easier to cross