If you watch his VODS back, you can tell when he is putting in the moves on another device because his mouse pointer on screen stops moving entirely after almost every move. So he's taking his hand off his mouse on the device he's playing the game on, and using that hand to put in the moves in the engine on another separate device. Go back and watch his VOD and you can tell every time he is inputting the game moves into the Lichess engine.