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.

> It would seem entirely unsurprising that cartels are monitoring border enforcement by drone

That's actually more plausible. I knew one person in "import-export" of illegal substance (Afghan/Iranian cannabis, he is probably out of work since the US left Afghanistan tbh, his clan was heavily involved with the british/US in both country since at least the Shah coup) and he explained how easy it is to smuggle a few tons of drugs and how hard it is to detect it, it seemed inefficient. Now surveillance drones, i buy that way more

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.