I am a pilot, I have training and some lungs-on experience with hypoxia. It is my opinion that this execution was botched; some combination of they did not sufficiently empty the atmosphere of oxygen and did not sufficiently remove CO2. The (rather shitty) article (or disorganized collections of opinions with nearly no structure and light on facts) mentions "a fitted mask" was used, which...yeah there's your problem. This probably did not provide enough isolation from the atmosphere and so he died of barely not enough oxygen.

Imagine instead walking into a classroom sized space that is full of 100% nitrogen. It's possible you wouldn't notice. In a space that large, your exhaled CO2 has enough room to diffuse away and not bother you. The fun part is, in this scenario you exhale O2 as well; breathing in this environment will kill you faster than holding your breath; being in that environment actually removes oxygen from your bloodstream. You're unconscious in less than a minute and dead in three or four. You don't struggle to breathe, nothing feels wrong, you get lightheaded, your vision narrows, the beginnings of a headache are probably your last earthly experience.

As the volume of air gets smaller, the O2 and CO2 exhaled by the condemned is more likely to be rebreathed, CO2 causes that "air is bad, need to breathe fresh air" feeling we're all familiar with, and O2 keeps you alive and conscious. One of those coffin-sized pods requires a good bit of gas flow to keep the atmosphere around the condemned's face pure.

If they put one of those medical masks they give you nitrous oxide or oxygen at the hospital with, just hooked up to a tank of N2...they probably weren't even completely excluding oxygen from the room.

Thats the thing about hypoxia, you don't struggle and your body doesn't react to reduced or non existent oxygen, its only when co2 starts to build up that people gasp and struggle to breathe. Watch the video in my edit above and tell me at what point Destin looked like he was in pain. But he absolutely was at risk of dying and quite rapidly in a scenario they were not trying knock him unconscious.

Our bodies simply do not react to nitrogen because 79% of our atmosphere is nitrogen. Our body expects that every gulp of air we breathe will be mostly nitrogen. Normally the co2 we exhale is heavier and falls towards our feet, and why sleeping with a blanket over your head can feel like you're suffocating.

The execution itself was flawed by not ensuring the victims head was elevated above the rest of his body and there was no method implemented to extract or remove the co2 he was exhaling. If he was in a proper pod or sealed chamber sitting up right with oxygen being removed as in the video, he would have lost consciousness in minutes to potentially seconds depending how quickly they could lower the o2 concentrations. Using a mask and having him lay on his back gave the co2 no where to go, ontop of him holding his breath which only amplified how his body reacted to rising co2 levels and sent him into panic.

Afaik, when performed correctly one will not struggle for breath, the body would be alive for some time but no consciousness remains at that point