You don't just need a GPS, you need a cellular radio, a SIM that works worldwide and the ability (or a strong enough receiver) to get both signals through whatever object they put them in. Given that most electronics have metal casings I find the idea that they pulled this off on the scale they claim to be quite suspect.
As a result I'd be willing to bet that they sent out very few of these devices and "extrapolated" out the data they claim and a proper statistical review of their methods and sample size would likely indicate that their claimed result is laughably wrong. If I had to bet I would wager they sent out less than 20 of these devices and have less than that got data back and used that to claim the percentage when there are hundreds of recyclers in the US.
The second part that bothers me is that similar claims to this were debunked in the past. There was a previous article claiming the US and Europe were shipping all their electronic waste to Africa when it actually turned out that only good working salable equipment was being shipped into Africa where it was re-purposed to be used productively and that the waste the article had photographed as an example was actually Africa's own electronic waste with none of Europe or the US.