Or they transported them in a Faraday cage, eg. a metal truck trailer.
Jamming cell signals is much harder because there are so many different bands and the transmitters are much more powerful. It requires bulky expensive hardware and some expertise to do it properly.
Plus you are more likely to be noticed if your vehicle is a rolling cell deadzone.
I listened to a talk a couple of years ago from a company that specializes in detecting GPS spoofers and one of the side effects was that they were able to watch people cheating their rental mileage agreements on box trucks via GPS jamming as they drove around London. The jammers would trip their sensors one after another as they drove down the road. There were hundreds of them operating in the city during the day.
You're right about jamming. I'm aware of several thefts in which covert cargo tracking or driver cell phone tracking was employed. At the end of the day they jam the signal, search the trailer, and discard the device or in the case of driver tracking..just turn it off. Tracking isn't the way.
There is very little the shippers can do to prevent it as well. It's up to the shipment broker to vet and identify identity theft of the trucking carrier when the load is being booked. Very few of them have any sort of security practices. The industry is woefully susceptible to even basic social engineering attacks.