[morepedantic]

By law, Social Security invests surpluses in US Treasury securities. So the US government "lends" money to itself, where it is spent that year on government programs for the taxpayers in that year. The securities are backed by taxpayers in the future.

For example, in 2005 Social Security had a 172B surplus, but the US government had a deficit of 318B. Social Security's 172B surplus was spent by the US government in the same year for government services on the people of 2005. The money was spent on voters, taxpayers, and citizens in 2005, but the liability was recorded against people who hadn't been born yet.

You've referred to a fiction, but reality is as I've stated. The US government loves creating fictions, but that doesn't mean we visit the Vietnam Police Action Memorial.