Yes, you are wrong.
If the cop merely held the man’s hand and sang a lullaby, the man still would have died.
The cop having his knee on the man’s neck did not cause his death. He did not die of asphyxiation. He died of ExDS.
If I ever walk by and see a cop with a knee on your neck and he remains like that until you die, I’ll make sure to tell the jury that you probably would have died anyway.