There are two parts to Twinbrook's fog.
1. The fog emitter, which is an effect available when building a world. In Twinbrook, they're place on the ground in certain spots (the cemetery, the swamps, etc.) to create fog on the ground. This is visible no matter what, and probably what you're seeing.
2. The fog in Priyansh's screenshot is a result of the sky parameters in TB's world files. It's a little hard to explain, but there is a file you can toy around with for a world that, among many other things, can control the sky conditions/clouds. In spite of a weather EP just being speculation, worlds can get cloudy/foggy days based on these settings already. I think that Twinbrook has a higher chance of showing a cloudy sky than, say, Sunset Valley. These chances are tunable, though I don't know how to tune them in an already-existing world.