It requires a custom 3D-printed "device" on the door, which would modulate the Wi-Fi waves being scattered around (by your router, laptop, etc) into a signal that would indicate "open" or "closed".
I think the point was that the large moving metal object (garage door) itself would mess with the signal enough to detect the state without any additional device…
I guess a first good experimental step would be to point a directional Wifi antenna in the direction of the door and monitor what it receives at what strength.
