They say if you bury an Ork in spring, by autumn you'll harvest a whole horde?
"Centurion!"
"Centurion!!"
Seeing the Centurion of their own Shadowmoon Wolves Legion crash heavily to the ground and lie still, and realizing the fight was over, the group of Astartes Space Marines who had been discussing and watching just a moment ago snapped back to reality. They didn't have time to marvel at the enemy battalion commander's bizarre outburst. Instead, they rushed through the mass-effect barrier and dragged him out of the unbreathable atmosphere of Planet Four-Four-Four as quickly as they could.
After all, although ordinary Astartes Space Marines had long surpassed normal humans in every measure of physical ability, and their bodies had been enhanced with genetic modifications and countless powerful artificial organs—making "superhuman" no exaggeration—they still couldn't last long if they exposed themselves without armor or protection to the vacuum of space or an atmosphere humans couldn't breathe. And that was even more true when they were injured.
Of course, perhaps the Primarchs were an exception?
As for how long the Primarch of their Legion or other Primarchs could survive in a vacuum or unbreathable environment, they had no idea.
After all, as far as the Astartes were concerned, the Primarchs and the Emperor of Mankind had already transcended the bounds of beings in the physical universe. They weren't people the likes of them could easily fathom or speculate about.
In any case!
Now that the fight was over, and they had seen the damage to the Centurion's power armor, they had to get him back and treat him as quickly as possible. This was only a duel, not a real battlefield, and they weren't fools. If they had the chance to save him, then of course they would do their best.
"How