From your link: "To the extent U.S. customer funds are held as cash, they are maintained in pooled custodial accounts at one or more banks insured by the FDIC. Our custodial accounts have been established in a manner to make available pass-through FDIC insurance"

So, should you have the desire to keep cash at Coinbase for some reason, it will actually be held at a real bank and FDIC insured.