The FDA might not see many opiate packages if most of the "hot leads" for which packages likely contain opiates are routed to the DEA for inspection and the FDA never sees them. Similarly for any other Schedule 2/3 drugs.
That could be why the FDA mostly sees "asthma" drugs. Though...I do believe that international orders would mirror domestic prevalence and lots of people need asthma medication. I myself have ordered asthma medications from India to save hundreds of dollars per month.
Anyways, the article states "The FDA said it found 33 packages of opioids and no fentanyl sent by mail in 2022". This was out of 53,000 packages inspected. However, the CBP reports finding 239 mail parcels with fentanyl at just JFK airport alone in 2018[0]. It's unclear how many packages the DEA has seized but it also sounds like a lot more than 33.