I would like to argue that it's not the point that there are other videos showing it, but that this is an educational video where somebody that doesn't know what they're doing might reasonably expect to be shown the proper way to do things. And plugging anything that isn't an actual plug into an outlet is definitely not the proper way to ground yourself. As others have said, a radiator, pipe or the metal casing of an appliance already plugged in would be.