Yep, you're entirely correct. But there's a reason those bonds are non-marketable; they're redeemable at face value at any time. They are the safest assets in the world, backed by the full faith and credit of the US government.
"By law, income to the trust funds must be invested, on a daily basis, in securities guaranteed as to both principal and interest by the Federal government. All securities held by the trust funds are "special issues" of the United States Treasury. Such securities are available only to the trust funds.
In the past, the trust funds have held marketable Treasury securities, which are available to the general public. Unlike marketable securities, special issues can be redeemed at any time at face value. Marketable securities are subject to the forces of the open market and may suffer a loss, or enjoy a gain, if sold before maturity. Investment in special issues gives the trust funds the same flexibility as holding cash." [1]
> The federal govt selling bonds to itself is a really strange form of savings.
Not at all. If you need something extremely reliable for decades, you essentially self-insure. There are no markets or assets the federal government could invest the Social Security trust fund in without creating market distortions, so it lends the funds to itself (which has the side effect of hopefully growing the economy, reducing the financial burden in the future).
If the trust fund runs dry, the federal government will top it up with general revenues. This would not be the end of the world.
[1] https://www.ssa.gov/oact/progdata/fundFAQ.html
EDIT: @ Retric (HN throttling has kicked in on my account, can't reply directly to your reply)
Too risky for a retirement trust fund that guarantees payments at nation-state scale.