I believe they're using CDC data, which states:

"Drug overdose deaths may involve multiple drugs; therefore, a single death might be included in more than one category when describing the number of drug overdose deaths involving specific drugs." https://www.cdc.gov/nchs/nvss/vsrr/drug-overdose-data.htm

Someone who overdosed after taking cocaine contaminated with fentanyl would be counted as a cocaine ODD.

The Oxycontin "reforms" caused the fentanyl crisis to begin with. People often moved onto heroin and fentanyl because pharmaceuticals were no longer accessible. The massive spike in overdose deaths begun after the decline in opioid prescriptions. See the Opioid Prescriptions & Opioid Overdose Deaths graph here https://drugabusestatistics.org/opioid-epidemic/