FSS LLP were allowed to pick a date. And the banks were asked to verify balance on that date. It's not like Tether can go back in time, and put money into the account on the chosen date.
It's like what some people do signing bitcoin messages. "As of this time, this address is under my control, and has this amount of bitcoins in it."
Then a few days later, the BTC moves to another address. But you can all see it in the blockchain.
banks could learn from blockchain and adapt some features.
Can not be that hard to make account information like balance and transaction public viewable. a additional online banking feature where account owners can control the access to this information by some kind of api.
I bet banks will do things like that to fight bitcoin.