There's nothing paradoxical about it. There's no such thing as a temperature gradient in a vacuum, there's nothing to hold or measure temperature against. And thus a vacuum is a really good insulator. Which is why a vacuum flask, which ultimately became one of Thermos' most well known products, is used to control temperature both in and outside the lab.
>>thwart+(OP)
Except a thermos has a really low emissivity, otherwise (if it had high emissivity), it’d be a poor insulator due to thermal radiation, the same reason why ISS’s radiators are much smaller than its solar panels.