Unless he was going to drop dead in that moment of whatever it was that ended up killing him, the people involved in the actions that exacerbated or expedited the death, killed him.
I don't disagree with that. But you'd be operating with the unfounded assumption that he would NOT have died without the officer's pressure on him. Prior to an autopsy or expert analysis, you could not accurately claim that.
Certainly, before people knew all the drugs he was on, and how he was struggling to breathe while in the car, it's not an unreasonable assumption to think he was killed, as it looked like it. But unlike the bullet that killed Thompson, being knelt on like that would not kill most people. So calling it unequivocally a killing prior to additional evidence, like you are, is unreasonable.