the obligations of the [Social Security] trust fund aren't tangible obligations fungible with other government debt. But at the same time they are obligations: they oblige the government to issue debt in the future, insofar as its tax revenue won't be able to cover its pension promises.
A trust fund = an obligation to borrow? If your uncle Louie told you he was setting up a trust fund for you, and its assets consisted of his obligation to borrow the money, how secure would you feel?
On the other hand, you may feel confident that Uncle Sam will always be able to borrow. Don't be so sure. A few months ago, one might have thought that Uncle Freddie and Aunt Fannie would always be able to borrow. That ended rather suddenly.
What do you like better about Sam? His balance sheet? His management team?