Going to Circle to redeem your USDC for FIAT USD is not the only way of receiving FIAT USD from USDC. You can also sell it for another asset, and sell that for FIAT. Or sell it for FIAT on an exchange.
The OP was about the safety of "holding" fiat on a single exchange vs USDC on your wallet.
Your point about non-custodial wallet risk is a misonemer as this is purely about can you trust yourself more than trust an exchange.