If you go to the section in the rulebook where it talks about Walls, it plainly says that LoS in never blocked to or from a flying creature by a Wall.
However, there is an error in the rulebook where it gives a visual example of how LoS is determined, it clearly shows a flying creature in a zone, but treats it as a non-flying creature.
If you go to the
Downloads section, there is a link at the top right of the website in the red bar, there is an updated rulebook which replaces the flying creature with a Hydra, to make it correct, on page 17.