Correct. This is very similar to holding stock securities. Most people who have retail trading accounts sign away this right to have a margin account. This means when you buy shares of AAPL, your broker is probably lending your shares to someone else since you chose not to hold the securities in your name but in "street name". The benefit to you is you get immediate access to margin vs waiting for the clearing process. The same is true in the futures market. Except we are not dealing with securities usually. US Treasuries are a security and like stocks, you can buy them and hold them in your name.
In the old days a big reason people did this is because in the 1980's, they paid 15% a year! This was how many FCMs made money back then. They took your cash and bought Treasuries, Pocketed the interest and lent you the money to trade. If you guys recall this is how PFG got in trouble. Except they didn't buy Treasuries, they bought higher risk securities. When those blew up, the customers were shocked. They shouldn't have been as all FCMs put this in their trade agreements which you sign. Which means it's even more important for you to take control of your funds.