This is a nonsense step. There is no meaning to the social security lending the government any money, as social security is the government
> Bob lends the government a trillion dollars. Bob sells this loan on the public market. Social Security happens to be the organization that buys that loan from Bob.
This scenario is just "The government buys back some of its debt, reducing its outstanding debt." The "Social Security" part is just a random detail that doesn't add anything.
(BTW, Social Security doesn't actually do this. They hold special bonds that are not the same ones available to the public.)