Wait a second. They used AMPRNet to manage these things? In here this kind of things are either hardwired to the internet or they use 3/4/5G uplink and while of course techinally possible either way to breach the system it’s a bit more difficult to find out proper IP’s and everything.
Once upon a time I had a task to plan a scalable system to display stuff on billboards and even replace printed ads on stores with monitors. The whole thing fell down as we couldn’t secure a funding for it, but I made a POC setup where individual displays had a linux host running and managing the display with (if memory serves) plain X.org session with mplayer (or something similar, it was about 20 years ago) running on full screen and a torrent network to deliver new content to them with a web-based frontend to manage what’s shown on which site. Back then it would’ve been stupidly expensive to have the hardware and bandwidth on a single point to service potentially few thousand clients, so distributing the load was the sensible solution. I think that even today it would be a neat solution for the task, but no one has put up the money to actually make it happen.