Well, they officially don't guarantee that they will redeem Tethers, but in practice they do, at least for the time being. So any withdrawal pressure would be de facto rather than de jure
Last time I checked, there were significant "limitations" (to be charitable... that you could only redeem holdings above $100K, only if you were a non-US person, and subject to 90-180 day holding periods.
People have also offered bounties for proof of people having redeemed Tether, and those bounties are still outstanding.
My suspicion is that if anyone has actually redeemed Tether, they are either institutional (and Bitfinex doesn't want to piss them off) or an insider/"friend".
Yes, USDC and USDT are different "securities".
But that's like saying that in the regular market that because Charles Schwab and Fidelity are competitors, they don't have a whole lot of cooperation - they do, because at that scale (and with the amount of arbitrage and speculation in crypto), you need to cooperate with your competitors, or you will be iced out.