Lack of oxygen doesn’t actually cause suffocation reaction. That is caused by high CO2 concentration. So breathing oxygen deficient gas does not cause distress as long as you can expel CO2. This is actually a major health hazard when handling compressed or liquefied gasses as if a nitrogen leak displaces the oxygen you won’t feel a thing until you fall unconscious and die.
I would guess that the pigs were sedated. Ethics standards usually require sedation in this kind of study whenever possible. I also imagine it would make it a lot easier to keep the oxygen pump their butts, and would also lower metabolic rates. Since the a key human use case is probably during extreme trauma surgery I think it would also be reflective of the expected use case in humans.
