I think this is only one part of the story for them. If 70% of their country is un-banked, many transactions are probably in cash, and there probably are a lot of undeclared sales and income that is not being taxed.

If they can really get their population using tools that would make it easier for the government to collect taxes, make it easier for tourists to visit and spend money, etc then the mining rewards are only one small part of their plan.

Of course none of this really needs to happen with crypto, they certainly could solve these problems in other ways, but I can't really blame them for trying this out.