Sure, to the technical definition of assets, but the point is that by definition fractional reserve banks don’t have the money on hand to pay out if all of their customers decided to withdraw at once.
This is a question of solvency. They were not Ponzis beforehand because the fair market valuation of their assets was correct. When the valuation changed, then they had the opportunity to become Ponzis, but things went a different way.
If banks had been forced to mark their assets mark-to-market, then they would have had to either 1) become insolvent, or 2) write-down customer deposits, or 3) take out huge amounts of loans to cover the difference.
The idea of TARP was to artificially mark up their assets so that they appeared to be solvent, with the Fed and the Treasury acting as the backstop in case of liquidity crunches or runs on the market. This never really panned out and the Fed just took the money and did 3 instead, and Congress was just "hey, fuck it, whatever".
2 is out of the question -- the formalism of the banking infrastructure created by the Fed and later the New Deal basically forbids saying "hey, you're taking a haircut on your savings account because you were really investing and investments may lose value".
1 was what some of them did, too, and then bigger banks came in and bought them up with borrowed money.