The nominal value is what matters because deposits are also nominally valued.
We don't live in a magical world where deposits (aka liabilities) are exempt from inflation and the assets that back them are not.
If you owe someone $1000 you owe them one thousand dollars. Not the value or purchasing power of one thousand dollars -- literally one thousand things called dollars.
So if you take $1000 in deposits, you buy $1000 in bonds, you wait until the bonds mature, and then your depositor withdraws $1000, you will be fine no matter if that happens in one year or a hundred years, no matter what the rate of inflation is. Your assets and liabilities will cancel out.
If your depositor tries to get their $1000 back before the bonds mature, you are screwed. That is what happened to SVB. If all of SVB's bonds had been mature by last Friday it would have been fine. Pretty much every article in the financial press about this fiasco has made that point.
It is amazing to see people twist themselves up in knots about opportunity cost and inflation when this is so basic. If liquid assets equal liabilities in the literal number of dollars, you are even, you are solvent, and your depositors get their money back.
> A loan today would need to be at below market rate interest in order to match up with the value of the bonds held to maturity.
Well, it's a government loan, so they can do that if they want to.