https://www.federalreserve.gov/newsevents/pressreleases/file...
They are only losing .1% of bond value instead of the difference between face and market value.
If the bonds were worth 80 cents on the dollar than they just traded 80 cents on the dollar for 99.9 cents on the dollar. A good deal if your balance sheet is in such poor shape.
If the fed values those bonds at face value (rather than what the market would pay today) and allows the bank to borrow money using those as collateral, then a bank could simply borrow from the fed using the older less valuable bonds as collateral and then buy new more valuable bonds.
The bank could then default on the loan and forfeit the original less valuable bond.
This would effectively be the fed giving free money to the bank.