It's different because the private company didn't issue the securities itself. The equivalent for a private company would be creating $3 trillion in bonds, "selling" the bonds to itself, then claiming to have $3 trillion.

In this analogy, they’d be different departments or subsidiaries with different balance sheets, so there’s no need for the scare quotes: they really are selling the bonds. And they didn’t issue $3 trillion in bonds, they issued $20 trillion and sold most of them to other parties on the same terms.

At the very least it indicates that the company prioritizes the finances of that one department much more than any other.

The other problem I have with this complaint is that it implies that things would be different if it didn’t hold treasuries. If it had $3 trillion in a bank account, would that make it “real”? The US government issues dollars too, and can devalue the currency at will, so it’s not really better.

Ultimately, it’s a commitment mechanism. If SS were paid out of the general fund, it could be cut off to fund other projects at will, as long as Congress were willing to piss off old folks. (Which is already a big obstacle, admittedly.) By funding benefits from a gigantic trust fund holding government debt, cutting off SS to fund something else would require imploding the world economy.