Removing a restraint can make subsequent movement possible, as opening a gate does. The critical question is whether an independently acting intermediary intervenes or whether the first act directly produces the loss.
The original discussion uses a historical example concerning the release of an enslaved person from restraint. Its instructional purpose here is to distinguish mediated from direct attribution, not to provide any contemporary rule about ownership of people.
Providing an opportunity is not automatically the same as directly performing every later act. A sabab by itself may be insufficient to establish liability.