The statements can also be parsed as "we have thousands of detections of an unknown number of drones that are being used by cartels to surveil the border in order to smuggle".

The number of drones vs. the number of detections is ambiguous. How detections are counted is ambiguous. Whether drones are physically moving drugs or part of an intelligence network is ambiguous.

It would seem entirely unsurprising that cartels are monitoring border enforcement by drone. That's not great, obviously. But different from "thousands of drones are carrying contraband into the US".

Lets be realistic, at best it actually means:

"We have thousands of possible detections of an unknown number of possible drones that are possibly being used by cartels to surveil the border in order to smuggle"

That's about 142 detections per day and yeah the big question is are they tracking unique drones (probably not) or radar contacts which appear and disappear? In which case the same drone flying around for 20 minutes and then being recharged would appear multiple times.

That number breaks down to 6 detections per hour across the entire boarder.

There's no reason to believe a single letter or number that comes from these people, and every reason to believe it's completely made up.