And if only we're true to the "original idea" (whatever that might be). As seen with lots of coins / exchanges it's been a front to do something else.
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.
Though that seems to have been the problem here.
The original idea with a 100% collaterilised peg was that the entire reserve would be cash. Somewhere along the way treasury bonds were considered cash equivalent. Which on the face of it seems sort of reasonable but clearly they do have a different risk and liquidity profile. This allows the centre consortium to earn a yield.
So I'm not sure I think a USDC is a dollar, but also I'm not sure it's particularly different to what banks do with deposits to earn a yield.
One difference is you can reinvest the same USDC to earn a yield while the underlying backing USD also earns centre a yield.