https://www.investopedia.com/terms/p/ponzischeme.asp
I think the point is that there are more structures out there like this than we think. Social Security for example. Just do a web search and you'll see it's a rather mainstream question. Also there are so many unfunded public pensions that could meet the definition.
Perhaps the difference is transparency. We know these public programs are currently in bad shape, but private schemes are completely opaque until they collapse.