That's what I meant by IOU. The treasury gives them the money, they return the bond, and it goes in the shredder. Of course it happens electronically these days, but before the internet there was a physical sheet of paper that was exchanged.

So instead of renewing/rolling over the bonds, countries can decide to collect the money owed and the US treasury can't say no.

If they're selling on the secondary market before the time is up, yes that's different. But if everyone decides to sell then supply outpaces demand and the value drops. Then nobody buys new bonds from the US and they still can't borrow, and have to default on their loans.

but they are bound to pay the specified amount on the specified date.

And since most institutional holders of US bonds will have a portfolio with bonds coming due on a regular and ongoing basis, it's not that hard for them to gradually divest while forcing the US treasury to cash out.