I agree it's a useful insight, but it's the only sentence in the entire article that isn't instead discussing the merits of lump sum vs regular payment. Saying that "it's the takeaway" from the linked article is insanity.
The Wikipedia page for Hitler includes the sentence:
The stock market in the United States crashed on 24 October 1929.
That doesn't make it the takeaway of the article!! If you want to make a case for something, bring the right evidence. As the researchers themselves have said, this study can't just be generalized to high-income countries.
In this experiment, external funding is paying for the handouts.
In a self-contained system, the same system/community providing the handouts would be generating the revenue for them (e.g., via taxation). Think of existing social welfare where "the system" generates the revenue that pays for the welfare programs.