The fact is George Floyd had a lot of fentanyl and meth in his system. The fentanyl level was 11 nanograms/ mill. Death by overdose has been certified in people with as little as 3 nanograms per mill. That is where the " over 3 times a lethal dose" number comes from. https://www.kare11.com/article/news/local/george-floyd/evidence-details-fentanyl-levels-george-floyds-body/89-70cf8552-1810-4462-a726-077b897e7378
However, as a habitual user, Mr. Floyd would have developed some tolerance for the drug. But no one can perform a test to say beyond a reasonable doubt that the lethal dose for Mr. Floyd was not 8 or 9 or 10 nanograms/mill.
The medical examiner said fentanyl and the pre-existing heart condition were contributing factors, but not the major cause.
The defense expert did not try to claim Mr. Floyd died from fentanyl overdose. He said Mr. Floyd dies from the combination of fentanyl and meth, the "speedball" combination acting on a less than healthy heart to cause a heart attack. He implied, but did not actually say, that without the drugs and pre-existing medical conditions that Mr. Floyd would not have died from the knee on his neck.
I think there is a lot of ambiguity the experts did not entirely clear up.