USDC -> USD always only ever worked during banking hours. Exchanges offering USDC <-> USD at any time have had to account for this and have enough reserves of USD and USDC themselves.
Would they really need USD reserves? Until the USD is withdrawn, it's just an entry on their internal ledger, and it can't be withdrawn outside business hours, and during business hours they can then exchange the USDC for USD.
It limits risk in the case of a depeg where you are unable to exchange USDC back into USD. You don't want all your company's assets to be exchanged into USDC.
This risk doesn't really depend on having USD reserves though, does it? If the company offers making the USDC <-> USD trades outside of business hours, it's taking the risk. (Of course, if it has the reserves it can be sure to remain solvent if that risk manifests.)
It's just the idea to limit how much USDC you are holding at one time. The more USDC you hold, the more risk there is. A reserve of USDC is just another way of saying that you have a max of USDC you are willing to hold.
