Saying that George Floyd didn't die from the triple-lethal amount of fentanyl in his blood, his severely bad heart, his COVID-19, and his hypertension, but that he died from "a lack of oxygen" is like saying, "Oh, he didn't die from the three shark bites. No, no. He died from a loss of blood."
Floyd was eventually unable to breathe (1) because he'd taken nearly 4 times the lethal dosage of fentanyl, a drug with a documented side effect of restricting or even stopping breathing at normal dosage, and (2) because he had COVID-19, one of whose symptoms is shortness of breath.
His heart stopped beating because he had a severe heart problem (90% narrowing of the right coronary artery and a 75% narrowing of the left coronary artery), because he had hypertension, and because he had trouble breathing due to the fentanyl and COVID-19.
If Floyd had not had these issues, he would not have died from 91.5 pounds of pressure being applied to his upper back or neck for 9 minutes. The prosecution's own breathing expert, Dr. Tobin, said the most weight that Chauvin applied with his left knee (the knee that was on Floyd) was 91.5 pounds. Early in the case, the medical examiner made it a point to note that tests had proved that a person of average health could still breathe even with 200 pounds applied to their back while in the prone position.
The medical examiner admitted, under cross-examination, that Floyd would not have died from being pinned for 9 minutes if he had not had the above-mentioned health issues.