Actually, Chauvin's actions did not have to be the only cause of death. Reckless actions only had to contribute in order for him to be guilty. The fact that there were drugs involved or other medical conditions merely establish that he may have been easier to kill than a healthier individual.

You only need to be very sure that what Chauvin did was reckless and unreasonable and also very sure that had Chauvin not knelt on his neck for nearly 10 minutes, that Floyd wouldn't have died at that same moment anyway.