You are reading it correct. The number is below zero. Not only do they have $0 available to back the savings deposits, they currently owe $20.9 billion.
This number will get far worse as the assets they acquire every Friday night during banks failures continue to depreciate in value. (Commercial Real Estate)
So who backs the FDIC? The Federal Government. The Federal Government is currently $13 trillion underwater and borrowing $2 trillion every year to stay solvent.
Who backs the Federal Government? The Federal Reserve. With a printing press.