Isenschmid also showed that Floyd's blood ratio of fentanyl to norfentanyl, the molecule fentanyl is broken down to once in the body, was lower than the average ratio both for people who died of overdoses and those arrested for DUI who lived. Overdose victims who die rarely have norfentanyl in their blood, since death often occurs before the body can break the drug down, he said
This paragraph contradicts itself. A ratio is a division. If there was no norfentanyl that would be division by zero. Thus we can see that if there is an average ratio overdose victims often (always) have norfentanyl in their system.
On the face of it this is a doomed argument. To illustrate consider the possible characterizations of nine minutes in metabolic terms.
If there is normally almost no norfentanyl in overdose cases and there was norfentanyl in Floyd, that implies that in overdose cases people die faster than Floyd died.
We know the difference in time from ingesting the drugs and death is roughly ten minutes. If ten minutes is enough time to process 5.6 ng/mL we're looking at 0.56 ng/(mL * min).
This allows us to set an absolute maximum time someone could take to die from fentanyl overdose since obviously they can't die if it took so long that they processed the drug into an inactive version. If 7 ng/mL is the lower limit of lethality then (7 ng/mL) / (0.56 ng/(mL * min)) = 12.5 minutes.
Keep in mind we know that anyone who survives longer than 12.5 minutes would have 5.6 ng/mL or more and the claim of the paragraph is that this is not observed.
Is it true that all fentanyl overdoses occur within 12.5 minutes? I would be very surprised. It makes far more sense that the lower the dose the longer you can live without dying and the more of the drug you will metabolize. If the absorption rate from whatever medium the drug came in roughly matched your metabolization of the drug you could have huge amounts of the by product build up while the active form of the drug suppresses your breathing slowly.
That is of course the basis of the field of anesthesiology.
That was logic, for those who don't feel comfortable with logic there is googling:
Mean fentanyl: 13.2ng/mL
Mean norfentanyl: 4.6ng/mL
Going to bring in some of my own "inexpert" math here: 13.2/4.6 = 2.87 = average ratio of fentanyl to norfentanyl.
George Floyd had a ratio of 11/5.6 = 1.96
So my fact check of the bolded paragraph is FALSE, and if it was said by a knowledgeable person it is a LIE.