The reason the Treasury doesn't expect to have to tap the $25 billion it is using as a funding backstop for the Fed's Bank Term Funding Program is because of the nature of the program.
The program is only accepting high quality federal government backed securities as collateral for the loans. While the collateral will be valued at par and not market value, the Fed has full recourse on the loan and recourse beyond the collateral. So if the loan isn't repaid, the Fed will have the collateral and can recoup any difference between market and par value from the bank's assets.
Note the $25 billion is just the backstop for the collateral. The lending program itself can be magnitudes larger than that.
Don't know why people are calling the Fed's BTFP quantitative easing by another name. The BTFP isn't purchasing securities. It is providing collateralized loans up to a year. It is basically a longer term discount window. When the loans are repaid, they'll come off the balance sheet. And the loans carry an interest rate of ~5%.
In any case most of the recent increase in the Fed's balance sheet has come from the vanilla discount window and extending credit to SVB and Signature in their receiverships. Less than 5% was the BTFP.