The Guinness in the can has no carbon dioxide in it yet. The widget is held closed by the pressure inside the can or bottle. When you open the can the pressure drops and the widget opens to let out a stream of (stuff) that adds the famous white head to the Guinness.
I used to work at a bar and when we ordered the gas to run the taps, we ordered regular CO2 for the non-Guinness beer, and “Guinness Gas” for the Guinness. Also, if we didn’t pour it slowly like the guy in the video did, our Irish and/or English patrons would send it back.