It's a perfect conductor of infrared radiation which is how we cool space stations without ambient air to remove the heat via convection.
Indeed. But radiative cooling in vacuum is much slower than conductive cooling per unit surface area (even just in air at sea level on earth, air being a fairly poor conductor) unless you manage to concentrate the heat in your radiator at a massive temperature that most materials can’t withstand.
And conductive heat transfer is what’s being measured in the context of this article.