Nowadays at least, wonder if they could just pick a few of the most popular unblocked HTTPS websites with a private messaging system a la Twitter DM. Email possibly too. A GitHub private repo would be perfect for that as dictatorships relaly hate to block it and lose all the IT value. Maybe at the time things weren't so simple with less HTTPS adoption.
I suspect the agents knew little about the comms tech and were deeply reassured of their security.
I think the reason they should have feared as soon as they began to become familiar with the website would be that they could find the form too easily, that the website was too small-- i.e. that their visits to it would make them conspicuous once something was noticed.
If it were Google and it was a standard Google, or some kind of Github chat that was like Signal but in JavaScript embedded in a website it would become much more reasonable.
I suspect the agents knew little about the comms tech and were deeply reassured of their security.