I restrict my interest to cases where I see a wrong verdict and this may be one of those but not necessarily. Your post feels like a personal attack.
In this case there is plenty of science showing a sudden death can occur. It is extremely rare for such a heavy sentence to be imposed where the victim was compliant at the outset, and there is no suggestion of premeditation. Maybe you have a parallel in mind.
But this is it: there's NOT "plenty of science showing a sudden death can occur" from what this man claims he did to Millane (i.e. manual choking). Heck, not even this man's own defence team (IIRC) tried to make the argument that Millane had died more-or-less instantaneously.
Instead, what all the reliable scientific evidence shows is that death from manual choking occurs only via two mechanisms: either a) constriction of one or both carotid arteries, causing starvation of oxygenated blood to the brain, leading to unconsciousness, coma, and death; or b) constriction of the trachea, causing insufficient (or no) air to get to the lungs, leading to asphyxiation and death. Mechanism (a) is relatively benign - the victim will cease struggling fairly quickly (sometimes a matter of seconds) as the brain responds to the sudden restriction or absence of oxygen by initiating unconsciousness; mechanism (b) is often violent and ugly, with the victim struggling often for well over a minute as they struggle to intake air. But - importantly - both mechanisms require well over a minimum of two minutes to cause death, and usually several minutes.
Put simply: this cannot have been an accidental death. If Millane had slipped into unconsciousness from constriction of her carotid artery/arteries, then - by both legal and ethical (and common-sense) definition - her consent immediately stopped at that point. And the man cannot have failed to notice that Millane had become unconscious: eyes closed, limp and unresponsive.
On top of all that, the entire purpose of these sexual breath-constriction "games" is to take the person to the point just before unconsciousness sets in, which is (apparently) the point at which maximum pleasure is derived. If the person falls into full unconsciousness, therefore, the "game" has necessarily gone too far. And what typically happens at that point is that the person doing the choking - having noticed that the person being choked has fallen unconscious - releases or relaxes the choke hold until the person regains consciousness. And then the process may be repeated. What the person doing the choking NEVER does is carry on with full pressure on the other person's neck for at least two, and as many as 6 or 7, minutes until the other person's brain has been so starved of oxygen that they die.
It's this man's right to apply to appeal both the verdict and the sentence. It's now up to the NZ Court of Appeal to adjudicate on whether he should be granted leave to appeal either or both of these things. I don't know what the man's defence team are arguing as grounds for appeal (verdict or sentence). One would hope the Court of Appeal will give proper consideration to his submission and return a fair and just adjudication as to whether or not he will be allowed to appeal.