[renonce]

> The original idea with a 100% collaterilised peg was that the entire reserve would be cash.

What is cash? Banknotes? You can't store 500 million pieces of $100 banknotes easily or safely.

Cash usually refers to deposits at accredited financial institutions like banks. Effectively this is an amount of money that the bank owes to Circle. The bank deposits money elsewhere, and the central place where all the money is distributed is the Federal Reserve Bank, the central bank of USA, that can never go bankrupt. OTOH, treasury bond is money that US Treasury owes to Circle, so they are not fundamentally different than cash, and in some cases it's even safer since US Treasury bonds are usually regarded risk-free.