Not my vid. I'm guessing the first time the camera was put on uneven in balance and that messed up the gyro. Either that or the less likely version is that the aerodynamics of the quad was altered by the exact placing of the camera and adjusting it even slightly made it work. IIRC the owner posted the video so you can simply ask them in a Youtube comment.
Mounting anything to the Mavic: I don't think it's a good idea to put stuff on the top of the drone, so close to the props. If I ever want to attach anything to it, I will use the two little legs with holes at the bottom and mount whatever at the bottom and hand catch it (as they did in the end of the video from 06:37).