Yes it is a vacuum and what does a rocket engine push against to propel itself? The vacuum kept the astronauts from being imeadiately flash frozen. That’s how cold 2.7K is.
The rocket exhaust pushes against the spacecraft. The vacuum doesn't allow the release of heat from the insulated CM. The bodies of the astronauts generate huge amounts of heat. That heat didn't go anywhere once the refrigeration units failed for lack of power. Their problem therefore also included getting back before they overheated and died. The International Space Station has the same problem on a much bigger scale, which it also solves with refrigeration units and huge heart radiators. So they were hot, not cold. Period.