In theory, the terraforming tools are not intended for use with the World Hub.
BUT, if you load the airport scenery, that will unload the current scenery (the in game one) and therefore also unload the auto terraforming.
On this example : My World Hub control tower flattened a hill - #2 by ASBNicolas
When you load the airport scenery, the terrain is hilly (only the terrain elevation apply here). But when you just start on the airport, the terrain is totally flatten by the rectangle.
Not sure if i’m clear… 