If messages are purported to be from a country's residents, it helps if they can be (eventually) traced to antennas there. That sounds fair to me. What seems surprising is to have so many in one place. And/or to have desks and manual operations (SIM cards) at that same location. I would have expected fewer antennas at once and unattended operation.

Except if this was next to a mall or large train station or something that might cover for the density of "phones".

There are ways to detect proximity to infrastructure and proxies... it's a lot easier/cheaper to just have boxes with like 50 LTE modems and pay someone some money to switch out sims once a day/week/whenever.

In other countries, SIM cards and mobile internet is stupid cheap.