Those surplusses were also the result of Baby Boomers paying lots of SS during their peak earning years.
If Clinton had saved that money instead of throwing it away on “Midnight Basketball”, etc., we’d be in much better shape today.
Those were the finishing touches that made them reportable as surpluses...though they weren’t, for the reason you cite.
The big CHANGE in money flow was a combination of spending limits - largely a legacy of ‘no new taxes’ Bush I demands, capital gain tax cuts bringing money into the active economy, and the welfare reform...all things that Clinton demonized, then claimed credit for after they worked. ...well, those, and slowing down Clintonian undermining of the economy, including killing Hillarycare.