Because anyone can create a fork of a chain the forks have to be authorized. The easiest way to authorize a chain would be to let the custodian (is that the right word?) of USDT decide which chains are authorized for USDT. If the custodian authorizes overlapping USDT then it is equivalent to creating an empty chain and creating USDT out of nothing to fill the newly created chain. In other words it would dilute the value of USDT.

This is an interesting problem because it shows a fundamental flaw in blockchains. They only have authority over themselves. No wonder cryptocurrencies and their advocates are so self centered. Because nothing that exists outside their chain (even other cryptocurrencies) is graspable!

It's not trustless because the trust (in the algorithm) is still there, the difference is that they minimized the need for trust to the absolute minimum. It's no longer about how much you trust but about what you trust.