they minimised the oxygen to the point he was nearly dead, then refreshed to revive him and then reduce it again over and over again, it was supposed to activate his mutation
So you've never heard of the part where a guy tortures another guy by slowly ripping out his fingernails until he's begging to have the whole finger cut off, and then his dick was cut off and the torturer ate a sausage in front of him?
After that the torturer "showed him kindness" by letting him sleep next to the dogs in the kennels, wearing the same clothes forever basically. Then, when his sister comes to rescue him, he screams for her to not take him away, because he's so psychologically broken and afraid of what the torturer will do to him if he escapes.
As far as my nollij of science goes, this scene was mildly unrealistic because if they reduced only the partial pressure of oxygen while maintaining ~1 atm he should still be able to breathe just fine, but if they reduced the overall pressure along with the PP of oxygen for as long as they did then he should have been bleeding through his skin.
The body uses CO2 concentration for when to breathe, not oxygen content. I counter your explanation by telling you to breathe helium from a balloon and exhaling but not take any breaths of normal air in between. You'll feel very lightheaded by the end of your second squeaky breath but no panic. That's why nitrogen is sometimes used in executions or euthanasia iirc.
You've never seen Game of Thrones?