40% isn't much in the grand scheme of things, but maybe they can reach higher reduction with more research/materials. Mass and power are pretty cheap for spaceX, so shipping more solar panels and a heap pump might not be a deal breaker.
Would e.g. a reduction of 90% in radiator area change the overall picture on the overall feasibility? I think not, it would still be ludicrous, but I'd be happy to be proven wrong.
The radiator area is probably not what they need to worry about that much as we thought. When the energy input comes from solar 100%, they just need to optimize the ratio of the sectional area facing the sun over the total surface area of the satellite. If the ratio is low enough, like a fin or cone shaped object, it will be harder to be hot.
>Heat radiation works better the higher the temperature?
The power output is proportional to T^4 according to the Stefan-Boltzmann law.