The pressure causes the asphyxia. I have never seen or heard of anyone (white or black) with a cop’s knee pressing on their neck for an extended period of time. It could happen for a shorter period by accident, but not that long. It’s completely contrary to policy and training for obvious reasons.
Police are also trained extensively on positional asphyxia issues. Notice how he appeared to be on his side. That is per policy cause you don’t want them face down. You also, of course, don’t want cops kneeling on their necks. It’s a horrible optic, no doubt about it.