Outer space works differently in the Marvel Cinematic Universe, especially the “Guardians of the Galaxy” films. Consider that, rather than putting on a spacesuit, Peter Quill has thus far managed to survive in a vacuum in his normal clothes and a disappearing helmet that doesn’t even cover his entire head. Like Captain Marvel’s space costume, it leaves the hair hanging out, which would destroy it. So we have to assume that certain items of protective clothing simply generate an entire mini-atmosphere around their wearers.
We’ve also seen Drax stand on top of the Guardians’ original spaceship, “Milano,” and man the cannons, his body seemingly unaffected by space. Drax is inhumanly tough, and we don’t know exactly how far out the atmosphere of Knowhere goes, so that gets a pass.
In this third film, we see spaceship windows crack open all over the place. Mantis starts to freeze as a spaceship exits the atmosphere, and Peter gets his face (and presumably body) knocked out of shape by exposure. There’s very little consistency in the amount of time it takes for the vacuum to do serious damage, and Peter, not unlike Arnold Schwarzenegger at the end of “Total Recall,” really looks like his contortions would cause lasting damage. Yet, he’s fine again almost immediately. Even though he supposedly lost his Celestial powers, it must be that Ego DNA.