[jonhendry]

Firstly, your link says nothing about customer deposits, it's about bank derivative holdings. Which aren't insured by FDIC.

But, yes, because it's pretty unlikely that all banks will fail at once, it's not necessary for the FDIC to have on hand a sum matching all insured US bank deposits.

There are also limits on coverage. If you keep $1 million in your savings account, you won't get all of it if the bank goes under.