It's also possible the suggestion is based on the concept of a country where 'filing taxes' is some optional or culturally odd thing so people need to be incent

It's also possible the suggestion is based on the concept of a country where 'filing taxes' is some optional or culturally odd thing so people need to be incentivised to do it at all. Or perhaps it's more thinking along the lines of 'someone who is a citizen' and measuring that by using tax filing as proof.