Hello Aurelien
The very last statement in the lesson answers your question. This statement says:
Active/standby failover does not use preemption. Once you enable the interface again, the currently active ASA will remain active.
This means that as soon as a failover takes place, once the standby ASA becomes active, it now plays the role of the primary ASA. If the first ASA comes back online, the roles are not automatically switched.
In order to return the active status to the original device, the simplest solution is to shutdown an interface on the currently active ASA that will cause a failure and will cause the other device to become active. Even though users shouldn’t perceive any network problems with such an event, it’s always best to perform this during low network traffic times or during maintenance windows.
I hope this has been helpful!
Laz