You don’t need to keep your entire body warm, you just need to keep enough of it warm for homeostasis, and ironically enough most modern clothing don’t address the greatest loss of heat, that active airflow system we call breathing. A simple COVID helps keep the heat tank that is your moisture from getting lost to the ambient air .

I mean, this outfit is clearly for cosplay, but keeping your sleeves short means being able to move your arms more freely and helps your skin from becoming damp swamp. It would have its uses even in cold weather.