You use arbitrary temps to prove at some temps it’s not as efficient. Ok? What about at the actual temps it will be operating in? We’re talking about space here. Why use 20 degC as the temperature for space?

He didn't use 20C as the temperature of space. He used the OP's example of comparing the radiative cooling effectiveness of a heat SOURCE at 90C (chosen to characterize a data center environment) and 20C (chosen to characterize the ISS/human habitable space craft).