No I'm arguing against a self imposed 'definition' of what near instantly is. To me that's things that take under five minutes, and things that render incurable harm in under two, brain death or body death need not occur in that space for me to classify it that way because medically the person is dying in the space of minutes.
So no, not arguing against the 'simple' logic, just the notion that it's definition is applied.
Besides short of lethal trauma I can think of scarce fewer quicker means of death.
I guess my medical background makes me disagree with you here. Also, common sense.
If someone dies in a car crash within a split second of impact, I would say that person died nearly instantly and at least the family could take solace in the fact that suffering would be minimal.
If a person died in agony within minutes of impact (lets use the under five minutes of your definition) after severe acute hemorrhage or respiratory arrest, I would most certainly NOT characterize that as a "nearly instant" death.
The definition is also relevant for situations of humane euthanasia for animals as well, as a nearly instant, painless death is clearly more humane than say, slicing vital vessels and letting them bleed out.
Or, to put it another way, the definition may seem arbitrary to an outside observer such as yourself, but if you were exposed to hard vacuum you would probably have a decent amount of time to think "I really wish this death was more instantaneous".

Tangentially related: I really hope we have the ability to throw someone out an airlock in Andromeda.