Probably just assumed that was the case due to common sense. I mean how many repulsor lift engines would you need to keep an ISD hovering like that? It makes me angry every time I see it, along with the ignoring of gravity wells for hyperdrive. The space whales in Rebels were just the worst. I mean they are SPACE whales not air whales. They should behave in air the same way our sea whales behave on land, and that is die.