In effect, the bonds I mentioned are just inverted loans. The Treasury takes in $1k from payroll taxes for these programs, issues a bond to the trust fund saying “I owe you $1k plus interest”, spends the $1k on whatever (I guess primarily the discretionary budget) and eventually has to somehow generate money to pay it back with interest.

In terms of whether or not bonds were “borrowed” - this wouldn’t exactly matter in a meaningful sense, but I’m not clear this ever actually happened in the first place. There was some claim about, when the trust fund was mixed with the general fund 1968-1990, maybe the government took bonds out of the program, but you have to remember that inside the government, that’s not something of value, that’s just an obligation the government has to pay to itself, it’s a big nothing burger.