Linkbait title from NPR. It's clearly not a secret government report, since if it was then it couldn't have been obtained via a FOI request.
It is a genuine concern, though, what would happen if US Treasury Bonds ceased to exist. There's a good argument to be made for the US not completely paying off its debts.
Well, the word secret has multiple meanings. I think it's fair to call this a secret memo, in a fairly broad sense of secret. But I agree that it's misleading, since secret has an additional sense specific to government documents.
It is a genuine concern, though, what would happen if US Treasury Bonds ceased to exist. There's a good argument to be made for the US not completely paying off its debts.