I'm not an expert on medical terminology; there may be a more accurate term to use to describe the cause of death. I'm only using the term in the colloquial sense, which (in my opinion) is a good description of what happened according to the video evidence.
But again, so what? Even if the death was technically caused by some other mechanism, it's still patently manslaughter when someone uses their knee to crush someone's neck for ~4 minutes until they stop moving. When that person continues using their knee to crush someone's neck for an additional ~4 minutes while ignoring the please from multiple bystanders to at least check for a pulse after he stopped moving, that demonstrates the intent and mens rea necessary to upgrade the charge of manslaughter to intentional murder.
The technicalities of a medical report are not relevant. If this was some sort of freak accident, the officer should have stopped what he was doing and checked vital signs. If he didn't intend to kill, the officer should have been very surprised and/or concerned when the person under his knee stopped moving and calling for help.
That's just it, though. According to the coroner's report, he wasn't strangled.