The difference is between exchanging tether and redeeming tether. Exchanging tether is easy as long as there’s a counterparty willing to buy your tether in exchange for USD.
But in principle you should be able to go to the “tether mint”, give them your tethers, and they redeem them 1:1 to USD. Apparently this is not very straightforward to do.
It would seem likely that the tether mint participates in some exchanges and buys or sells to keep the price stable.
This theory is backed up more by the fact the tether price history matches with a script in someone's basement that occasionally crashes and it takes a few hours to repair, during which the price wanders before being clamped 1:1 again.
That's functionally identical to going to the mint directly, and means the mint doesn't require any customer service etc.
But in principle you should be able to go to the “tether mint”, give them your tethers, and they redeem them 1:1 to USD. Apparently this is not very straightforward to do.
This podcast episode covers the topic well: https://anchor.fm/cas-piancey/episodes/Tether-A-Stable-Discu...