Xorkoth
Bluelight Crew
Fentanyl is super easy to overdose and die from even at low levels and addicts and regular users with a tolerance to opiates overdose on low doses of Fentanyl all the time, and random amounts of Fentanyl used by addicts or polydrug users/addicts on their own is not the same as a controlled and known amount used in a hospital for anesthesia by surgeons, doctors, or anesthesiologists.
The estimated lethal dose of fentanyl in humans is 2 mg. The recommended serum concentration for analgesia is 1–2 ng/ml and for anaesthesia it is 10–20 ng/ml. Blood concentrations of approximately 7 ng/ml or greater have been associated with fatalities where poly-substance use was involved.
Street fentanyl has hot spots and some is stronger than other batches, so its notoriously deadly. However, if someone took a very small dose of a very potent street fentanyl and died, their toxicology report would show lethal blood concentrations. While the source you found (first google result I got) does say that concentrations as little as 7ng/mL have been lethal in drug combinations, that is specifically with other substances that depress breathing or potentiate opioids, notably benzodiazepines or other depressants, not meth. But Floyd had small amounts of meth as well as fentanyl in his blood so that doesn't really pan out. And most importantly, those concentrations are for non-tolerant people. There are case studies of people who have been addicted to opioids for many years who can take 20 or more times the lethal dose for a non-tolerant individual and be fine. Opiate addicts regularly take doses that would have killed them before they got addicted, it's just how it works. Now I don't know how much tolerance Floyd had, but chances are, if he was such a huge drug addict, he had at least some, if not a lot. So 11ng/mL is simply not enough to cause him to die.