Social Security is a government program like any other. The delineation of tax types is just branding to make people like it more, it isn't economically meaningful. When the payroll tax is higher than the SS spend, the government just spends the money. (These bonds are IOUs for all the times they do that.)
It's like saying that it's meaningless for you to loan yourself money, because it is (except when there is some other, external reason that it carries meaning, not by rules you made yourself.) We constantly ignore these internal self-debts for people and for companies.
https://www.cbpp.org/research/recommendation-that-presidents... notes
> Most economists agree that the debt held by the public is what really affects the economy. As the Congressional Budget Office stated in its June 2009 report on the long-term budget outlook, “Long-term projections of federal debt held by the public, measured relative to the size of the economy, provide useful yardsticks for assessing the sustainability of fiscal policies.” In contrast, “gross debt . . . is not useful for assessing how the Treasury’s operations affect the economy.”