I don't think this is true. During the surplus years the savings vehicle was US bonds. So social security lent the US government money.
Now that we are not in a surplus situation, we are using money from those bonds to pay benefits (we are also using SS revenue of course, only part of the payouts come from the bonds).
So if the program were cancelled, the bonds could revert back to the US government - in other words a bunch of debt would be forgiven and would not need to be paid out. Deficit (and debt) reduced.