There is a trust fund. It is filled with interest bearing T-bills that are only redeemable by the USG. The problem is that the USG must come up with the money to redeem them out of the general fund. The SSTF really is just a manifestation of the full faith and credit of the USG to pay the SS benefits. And by 2036, the SSTF T-bills will be gone and SS will only be able to pay about 75% of the benefits.
Milton Friedman said it best:
“The only way that the Treasury can redeem its debt to the Social Security Administration is to borrow the money from the public, run a surplus in its other activities or have the Federal Reserve print the money -— the same alternatives that would be open to it to pay Social Security Benefits if there were no trust fund. But the accounting sleight-of-hand of a bogus trust fund is counted on to conceal this fact from a gullible public”