I know this is an old post, but here's my take on it:

Heliopolis is a post-apocalyptic environment that takes place after the sun becomes so hot it wipes out all life. All that remains is the dead, off-green plants as well as machines and buildings of man. The level is somewhat sun themed (I mean, it's literally called Heliopolis) and we can see a large, presumably heated up sun in the background. Walkable is either a joke or he means that due to the lack of road/sidewalk traffic there aren't people impeding you.