Are they streaming textures?
People who get really in to flight sims are often very dedicated hobbiests who end up with a pretty wild array of specialized equipment and peripherals just to play. As the article says, what the game is streaming in is immense amounts of geospatial and photogrammetry data - aerial and satellite images processed to provide detailed 3d maps of the real world for the player to fly around in. It sounds like they might be getting in to wacky things like real-time weather modelling, too, which... woooooah.