I think it's more likely to be a measure of heat absorbed, rather than heat transferred across a boundary. (The Telegraph's wording is "an energy absorption rating"). Starlite is apparently ablative; if this means an endothermic phase change -- the organic polymers vaporizing, without combusting -- it could absorb an extremely large amount of heat. A space shuttle tile is non-ablative (I think?), and can absorb heat only by increasing temperature.