Since the guy died of fentanyl, meth and a heart attack as well as other factors the prosecution will have their hands full proving that the cop caused it all.
Nobody is saying he caused it all. But his actions contributed to it, per the medical examiner. Take any of the three factors out of it and Floyd likely doesn't die there. Combine the three and it killed him. So all the prosecutor has to prove is the cops actions contributed to the death and a conviction should follow.