Found in the article “Excited Delerium”. I am tending toward not blaming the officers.
“Based on these findings, PMRP may result in a transient pattern of restricted pulmonary function, but the lack of evidence for hypoxia or hypoventilation suggests that factors other than body positioning appear to be more important determinants for sudden, unexpected death. Nonetheless, respiratory muscle fatigue resulting from exertion and struggle against restraints (exertion vs. position asphyxia) cannot be excluded nor can potentially fatal pre-existing problems with central cardiac output, oxygen saturation, or oxygen use.6,26,28,34”
Because physical force was being used at the moment of death, other medical factors don't absolve the police of responsibility.
Floyd died while the cop was kneeling on his neck, regardless of the actual cause. If he did indeed die from unrelated causes, then that's the officer's bad luck.
As another poster pointed out, shooting someone who has jumped from the Golden Gate bridge makes you a murderer, even if the ME rules that they died from the fall.