There are two ways to look at this. Both can be supported by strong arguments.
One, the government increased payroll taxes to save for the coming crush of baby boomers retiring. They collected more in taxes than they needed to pay out in benefits. The surplus lasted from the late 1980's until recently, there is something of a dispute as to when the collection of taxes exceeded the benefits paid out. I don't remember the details.
This by the way is the largest tax hike in history of the US. The government used the money from the tax hike and issued bonds for the amount, which they had to do by law, and gave the bonds to the Social Security Administration. These are the bonds that George W. Bush called worthless.
Or two, you can argue that it is meaningless for the federal government to say that they are saving dollars in any type of scheme like this because the federal government is the entity that creates dollars to be used as money that has value. That the federal government can create as many dollars as they need whenever they need them.
But is also saying this, that it makes no sense to say that the government is saving, the opposite side of the coin has to be equally true, that the federal government can't be in debt in dollars. The problem is that nearly everyone who is saying the federal government can't save also insists that the federal government is bankrupt because the national debt is so high. At least they say this when the Democrats are in charge. They don't seem to worry about the national debt when they pass tax cuts for the very rich or they increase defense spending or want to build that wall.
I go with two. The federal government can't go bankrupt when the debt is in dollars. It is meaningless therefore for the federal government to say that they are saving dollars in a trust fund or to say that they are in debt in dollars. They can never say that they are saving dollars for future generations or that they are running up debt that will be passed to future generations in dollars.
We are passing on a large debt to future generations but it isn't one of dollars, it is one of a crumbling infrastructure, inadequately educated children, never ending war, and our failure to solve poverty, racism and misogyny, all of which we were capable of doing.
That being said, the issuing of the bonds demonstrates that the intent of the tax hike was to fully fund Social Security from the general fund and from the taxes that flow into that fund, and not to see benefits cut or the payroll tax raised.