They didn't roll anything back. That ledger, with the loss, exists today unchanged. What did happen was a superset of users decided to create a new ledger to r

They didn't roll anything back. That ledger, with the loss, exists today unchanged. What did happen was a superset of users decided to create a new ledger to run in parallel, containing transactions up until but not including the loss. The market determined the new ledger to be more valuable