If I take a loan from the bank for $1000, but also have $1000 in a bank account in another bank, then my debt is 0. Social Security is a bit like the second bank account.
That is correct.
In the case of social security, you also have to account the total amount that social security owes to its contributors.
Because at the end of the day, Social Security has to pay out benefits, this debt isn't just pointless accounting fiction.
"Social security owns government debt" very clearly communicates that the government indiscriminately defaulting on loans will mean that social security beneficiaries will have to take a haircut - just like if I replace the words "Social security" with "XYZ pension fund." Just because the government runs this particular pension fund doesn't mean that it's all that much different, accounting-wise from a private one.