That's because Social Security has been running a surplus for many years (until 2010). The question is whether the US government, which already spent that surplus, but gave a bunch of bonds as promissory notes to repay, will actually do so. Payroll taxes were meant to prepare for the baby boom retirement, and were enough to do so, but that money is gone, not in a "lock box".
It's also been completely expected since the 1980s, so it's weird that this is now being portrayed as some kind of a problem. Reagan proposed an increase in payroll taxes, so that Social Security would run a large surplus for ~25 years, which would then fund the baby-boomer retirement. As far as cash-flow goes, that surplus went into the general treasury during the surplus years, and now it is the general treasury's obligation to repay it to Social Security out of general revenues for the next ~30-40 years.
It's possible people will choose not to do so, but that's not a matter of Social Security being insolvent. That's a matter of people simply not wanting to keep the promises, because they want lower taxes or something. If Social Security is not repaid, then payroll taxes will be retroactively converted to a regressive general income tax, where only income under $114k is taxed, which would be rather bad.
