The article you've linked says it didn't go far (buggy, vulnerable, difficult to use, test deployment in 2020). Also, Google Play store's reviews say the same.

It's only natural that when a public servant want to make sure their message actually reaches the recipient, they would rather use a solution that 100% works. That's not malice, it's common sense.

I think the question is not whether it's possible to set up in theory.

The question is whether it's possible to actually deploy and maintain the solution in reality to millions of concurrent users in hundreds thousands of disjoint organizations in thousands of municipalities all over the country. It would require a constant supply (due to organizational churn) of highly qualified tech people who'd willing to be working years for low-five-figures salary in a rigid ineffective governmental structure, surrounded only by non-tech-savvy people. These tech people should also be saints so that they don't embezzle half the budgets while trying to set things up. Also the top visionary who'd push for this multi-year project should not be subject to election cycles.

I honestly think that this level of concentration of long-term effort might be impossible in a country with more than 10 million people or so.

How is it different from WhatsApp (apart from being worse feature-wise and not many people using it)? As far as I remember, WhatsApp uses Signal's encryption, so technically they're the same level of protection?