The article describes a totally enclosed SMPS similar to the one used on laptop computers. It has no fan and no (or minimal) ventilation. The heat is essentially trapped inside and can only escape through the plastic case which is a relatively poor thermal conductor. The metal foil is to distribute the heat more evenly to make best use of the whole plastic surface and more importantly, to prevent the plastic deforming close to hot components. It isn't a heat sink as such because it isn't attached to anything, it just does as the name suggests, spreading the heat around a bigger area. The bonus is, as FvM states, it also helps to shield EM emissions.
Brian.