The article spends a lot of time explaining how these IOUs can be anonymous, but I don’t understand how it can trustlessly interact with another blockchain. If

The article spends a lot of time explaining how these IOUs can be anonymous, but I don’t understand how it can trustlessly interact with another blockchain. If I send some bitcoin to it, what guarantees that I will get a valid ‘IOU’ back, in an atomic way? If I cash in an IOU, what guarantees that I will receive the expected bitcoin deposit? Do I have to trust that this new entity won’t just run off with my money?